Scientific Output

Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

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  • 2020 • 197 α-Aminoisobutyric acid-stabilized peptide sams with low nonspecific protein adsorption and resistance against marine biofouling
    Beyer, C.D. and Reback, M.L. and Gopal, S.M. and Nolte, K.A. and Finlay, J.A. and Clare, A.S. and Schäfer, L.V. and Metzler-Nolte, N. and Rosenhahn, A.
    ACS Sustainable Chemistry and Engineering 8 2665-2671 (2020)
    A series of low fouling peptide self-assembled monolayers (SAMs) was developed to understand how the effects of subtle sequence alterations determine the properties of peptide-terminated SAMs and settlement and adhesion of two model fouling organisms, the green alga Ulva linza and the diatom Navicula perminuta, and adsorption of two different proteins, fibrinogen and lysozyme. Insertion of the bulky, nonproteinogenic amino acid α-aminoisobutyric acid (Aib) was examined for how it affects the peptide surfaces and performance in the assays. By exchanging the serine (S) of the sequence (SGKGSSGSS) with alanine (A), we slightly altered the hydrophilicity and found reduced fouling by N. perminuta. The inclusion of Aib residues resulted in surface structural changes of the peptides from a mixture of β-sheet/random coil to strictly random coil and a decrease in the overall packing density by about 17-37%. Notably, these changes had little effect on the ability of the surface to resist nonspecific adsorption of fibrinogen and lysozyme and attachment of N. perminuta. The sequences containing Aib were 50-84% better than without Aib against the settlement of the zoospore of U. linza. Furthermore, the inclusion of Aib helped to create peptides that were 100% resistant against enzymatic degradation by trypsin, whereas the peptides without Aib were 95% degraded after 4 h. © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acssuschemeng.9b05889
  • 2020 • 196 Fabrication of a novel and highly selective ion-imprinted PES-based porous adsorber membrane for the removal of mercury(II) from water
    Esmali, F. and Mansourpanah, Y. and Farhadi, K. and Amani, S. and Rasoulifard, A. and Ulbricht, M.
    Separation and Purification Technology 250 (2020)
    Herein, poly(ether sulfone) based ion imprinted membranes (IIM) were prepared through phase inversion, using ion imprinted polymer (IIP) particles obtained by radical copolymerization of acrylamide, acrylonitrile and ethyleneglycoldimethacrylate along with a template of Hg(II) complexed with bathophenanthroline (BPh). Optimization of the ability for Hg(II) removal from water and pure water flux of the IIM were investigated through Central Composite Design (CCD) combined with Response Surface Methodology (RSM). Accordingly, the optimized factors were obtained as IIP percentage of 2.5 wt% used in membrane preparation, as well as trans-membrane pressure of 0.19 bar, pH 7.95 and Hg(II) concentration of 4 ppm during filtration through the membrane. Using the optimum parameters, the removal percentage and flux of IIM were about 98.1% and 37.5 kg/m2 h, respectively. The maximum adsorption capacity of IIM was 432 mg/m2 (or 21.6 mg/g), almost four times higher than that of non-imprinted membrane (NIM; 105 mg/m2) (or 5.25 mg/g) which was prepared using copolymer particles prepared without the Hg(II) template. The IIM showed a high selectivity toward Hg(II) ions compared to other metal ions and could be effectively recycled for at least 6 times without any major loss of adsorption capacity. The synthesized imprinted membranes have demonstrated considerable potentials to selectively separate mercury(II) from simulated industrial wastewater. © 2020 Elsevier B.V.
    view abstractdoi: 10.1016/j.seppur.2020.117183
  • 2020 • 195 Design of Complex Solid-Solution Electrocatalysts by Correlating Configuration, Adsorption Energy Distribution Patterns, and Activity Curves
    Löffler, T. and Savan, A. and Meyer, H. and Meischein, M. and Strotkötter, V. and Ludwig, Al. and Schuhmann, W.
    Angewandte Chemie - International Edition 59 5844-5850 (2020)
    Complex solid-solution electrocatalysts (also referred to as high-entropy alloy) are gaining increasing interest owing to their promising properties which were only recently discovered. With the capability of forming complex single-phase solid solutions from five or more constituents, they offer unique capabilities of fine-tuning adsorption energies. However, the elemental complexity within the crystal structure and its effect on electrocatalytic properties is poorly understood. We discuss how addition or replacement of elements affect the adsorption energy distribution pattern and how this impacts the shape and activity of catalytic response curves. We highlight the implications of these conceptual findings on improved screening of new catalyst configurations and illustrate this strategy based on the discovery and experimental evaluation of several highly active complex solid solution nanoparticle catalysts for the oxygen reduction reaction in alkaline media. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/anie.201914666
  • 2020 • 194 The synergistic effect of heterostructured dissimilar metal-organic framework thin films on adsorption properties
    Wang, Z. and Wannapaiboon, S. and Henke, S. and Paulus, M. and Rodewald, K. and Rieger, B. and Fischer, R.A.
    Journal of Materials Chemistry A 8 12990-12995 (2020)
    A metal-organic framework (MOF) heterostructured thin film of 3D Cu3btc2 on 2D SURMOF-2 was developed for VOC adsorption. This heterostructured thin film shows higher VOC storage capacity than the two components and a counter-intuitive uptrend of adsorption ability (dimensionless normalized storage capacity) with increasing the size of VOCs. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9ta10264g
  • 2020 • 193 Acidity enhancement through synergy of penta- and tetra-coordinated aluminum species in amorphous silica networks
    Wang, Z. and Li, T. and Jiang, Y. and Lafon, O. and Liu, Z. and Trébosc, J. and Baiker, A. and Amoureux, J.-P. and Huang, J.
    Nature Communications 11 (2020)
    Amorphous silica-aluminas (ASAs) are widely used in acid-catalyzed C-H activation reactions and biomass conversions in large scale, which can be promoted by increasing the strength of surface Brønsted acid sites (BAS). Here, we demonstrate the first observation on a synergistic effect caused by two neighboring Al centers interacting with the same silanol group in flame-made ASAs with high Al content. The two close Al centers decrease the electron density on the silanol oxygen and thereby enhance its acidity, which is comparable to that of dealuminated zeolites, while ASAs with small or moderate Al contents provide mainly moderate acidity, much lower than that of zeolites. The ASAs with enhanced acidity exhibit outstanding performances in C–H bond activation of benzene and glucose dehydration to 5-hydroxymethylfurfural, simultaneously with an excellent calcination stability and resistance to leaching, and they offer an interesting potential for a wide range of acid and multifunctional catalysis. © 2020, The Author(s).
    view abstractdoi: 10.1038/s41467-019-13907-7
  • 2019 • 192 Comprehensive Methodology for the Investigation of Mercury Adsorption on Activated Carbons
    Ambrosy, J. and Pasel, C. and Luckas, M. and Bittig, M. and Bathen, D.
    Chemie-Ingenieur-Technik 91 1874-1884 (2019)
    In mercury adsorption on activated carbons both physisorptive and chemisorptive mechanisms play a role. The systematic investigation of Hg0 chemisorption is difficult because equilibrium capacities cannot be determined due to slow adsorption mechanisms. Therefore, the present publication suggests a three-step approach: 1) Breakthrough curves are used to assess the dynamics of Hg0 adsorption. 2) The contributions of physisorption and chemisorption can be distinguished by coupled adsorption and desorption experiments. 3) Temperature programmed desorption (TPD) experiments are performed to get information about specific chemisorptive binding sites on the surface. This approach was tested on four characteristic examples of impregnated and non-impregnated activated carbons. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cite.201900068
  • 2019 • 191 Temperature Swing Adsorption in Natural Gas Processing: A Concise Overview
    Berg, F. and Pasel, C. and Eckardt, T. and Bathen, D.
    ChemBioEng Reviews 6 59-71 (2019)
    To enable the technical use of natural gas, efficient gas processing is essential. Various components such as water, sulphur compounds, carbon dioxide, nitrogen, and heavy hydrocarbons must be removed. Temperature swing adsorption (TSA) is a commonly used way of removing some of these components. This paper describes where TSA is used in the natural gas treatment process and outlines the application of commercial adsorbents in TSA plants. The state of research on adsorbents and process control in TSA plants in natural gas processing is discussed. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cben.201900005
  • 2019 • 190 Anomalously Low Barrier for Water Dimer Diffusion on Cu(111)
    Bertram, C. and Fang, W. and Pedevilla, P. and Michaelides, A. and Morgenstern, K.
    Nano Letters 19 3049-3056 (2019)
    A molecular-scale description of water and ice is important in fields as diverse as atmospheric chemistry, astrophysics, and biology. Despite a detailed understanding of water and ice structures on a multitude of surfaces, relatively little is known about the kinetics of water motion on surfaces. Here, we report a detailed study on the diffusion of water monomers and the formation and diffusion of water dimers through a combination of time-lapse low-temperature scanning tunnelling microscopy experiments and first-principles electronic structure calculations on the atomically flat Cu(111) surface. On the basis of an unprecedented long-time study of individual water monomers and dimers over days, we establish rates and mechanisms of water monomer and dimer diffusion. Interestingly, we find that the monomer and the dimer diffusion barriers are similar, despite the significantly larger adsorption energy of the dimer. This is thus a violation of the rule of thumb that relates diffusion barriers to adsorption energies, an effect that arises because of the directional and flexible hydrogen bond within the dimer. This flexibility during diffusion should also be relevant for larger water clusters and other hydrogen-bonded adsorbates. Our study stresses that a molecular-scale understanding of the initial stages of ice nanocluster formation is not possible on the basis of static structure investigations alone. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.9b00392
  • 2019 • 189 Adsorption of the Inhalation Anesthetic Isoflurane from Dry and Humid Atmosphere
    Bucher, D. and Pasel, C. and Luckas, M. and Bathen, D.
    Chemical Engineering and Technology 42 1268-1275 (2019)
    For environmental and occupational safety reasons, the inhalation anesthetic isoflurane must be removed from indoor air in hospitals. The present work investigated experimentally the coadsorption of isoflurane and water on four different commercial adsorbents: two activated carbons and two zeolites. Adsorption isotherms from dry and humid atmosphere are shown and discussed, i.e., mixture data are compared with those of the pure substances. For all four adsorbents the dominant parameter regarding structural properties is the pore size distribution. In the case of the zeolites, the surface chemistry, especially the aluminum content, plays a role, too. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/ceat.201900072
  • 2019 • 188 Adsorption and dissociation of iron phthalocyanine on H/Si(111): Impact of van der Waals interactions and perspectives for subsurface doping
    Geisler, B. and Kratzer, P.
    Physical Review B 99 (2019)
    The adsorption of iron phthalocyanine (FePc) on the passivated H/Si(111) surface is explored from first principles. We find that the organic molecule is predominantly physisorbed with a distance to the surface of 2.6±0.1Å and an adsorption energy of 1.55±0.1 eV but also exhibits sizable resonance with the underlying substrate. This establishes the present system as interesting mixed covalent-van der Waals-bound test case, which we use to compare the impact of different approaches to van der Waals interactions. Spin-polarized scanning tunneling microscopy (SP STM) images are simulated, selectively accessing different molecular orbitals via the applied bias voltage in the spirit of scanning tunneling spectroscopy. Comparison with experimental STM images reveals very good agreement. We report a significant magnetic contrast exceeding ±1Å in the SP STM images for -2 and +1.5 V. Aiming for a magnetic functionalization of Si for possible spintronics applications, magnetic moments and binding energies of different (transition-metal) atoms in the center of the Pc ring are presented, which particularly show that Fe is strongly bound in the molecule (about 9.6 eV). Finally, we discuss different mechanisms for subsurface Fe doping by room-temperature FePc deposition and point out two feasible reactions. Concomitantly, we identify the crucial role of a preceding destabilization of FePc, for instance, by preadsorbed H atoms, which subsequently strongly stabilize the final state of the reaction. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.155433
  • 2019 • 187 Molecule-Metal Bond of Alternant versus Nonalternant Aromatic Systems on Coinage Metal Surfaces: Naphthalene versus Azulene on Ag(111) and Cu(111)
    Klein, B.P. and Morbec, J.M. and Franke, M. and Greulich, K.K. and Sachs, M. and Parhizkar, S. and Bocquet, F.C. and Schmid, M. and Hall, S.J. and Maurer, R.J. and Meyer, B. and Tonner, R. and Kumpf, C. and Kratzer, P. and Gottfried, J.M.
    Journal of Physical Chemistry C 123 29219-29230 (2019)
    Interfaces between polycyclic π-electron systems and metals play prominent roles in organic or graphene-based (opto)electronic devices, in which performance-related parameters depend critically on the properties of metal/semiconductor contacts. Here, we explore how the topology of the π-electron system influences the bonding and the electronic properties of the interface. We use azulene as a model for nonalternant pentagon-heptagon (5-7) ring pairs and compare it to its isomer naphthalene, which represents the alternant 6-6 ring pair. Their coverage-dependent interaction with Ag(111) and Cu(111) surfaces was studied with the normal-incidence X-ray standing wave (NIXSW) technique, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, UV and X-ray photoelectron spectroscopies (UPS and XPS), and density functional theory (DFT). Coverage-dependent adsorption heights and spectroscopic data reveal that azulene forms shorter interfacial bonds than naphthalene and engages in stronger electronic interactions with both surfaces. These differences are more pronounced on Cu. Increasing coverages lead to larger adsorption heights, indicating bond weakening by intermolecular repulsion. The extensive DFT calculations include dispersive interactions using (1) the DFT-D3 scheme, (2) the vdWsurf correction based on DFT-TS, (3) a many-body dispersion (MBD) correction scheme, and (4) the D3surf scheme. All methods predict the adsorption heights reasonably well with an average error below 0.1 »Å. The stronger bond of azulene is attributed to its nonalternant topology, which results in a reduced highest occupied molecular orbital (HOMO)-lowest occupied molecular orbital (LUMO) gap and brings the LUMO energetically close to the Fermi energy of the metal, causing stronger hybridization with electronic states of the metal surfaces. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.9b08824
  • 2019 • 186 Functionalization of Ultrafiltration Membranes for Integration of Specific Adsorber Properties [Funktionalisierung von Ultrafiltrationsmembranen zur Integration von spezifischen Adsorbereigenschaften]
    Koch, D. and Ulbricht, M.
    Chemie-Ingenieur-Technik 91 1129-1134 (2019)
    By integrating a water-soluble polymer, which has the ability to complex heavy metal ions into ultrafiltration membranes, the separation process could be enhanced to enable also filtration of these species. In this work, a membrane and an adsorber polymer were functionalized with complementary reactive groups so that the adsorber polymer could be immobilized in the porous support layer of the ultrafiltration membrane via click reaction. The separation performances and membrane characteristics of the synthesized membranes are comparable to those of conventional UF membranes. © 2019, Wiley-VCH Verlag. All rights reserved.
    view abstractdoi: 10.1002/cite.201900046
  • 2019 • 185 Cellulose acetate/layered double hydroxide adsorptive membranes for efficient removal of pharmaceutical environmental contaminants
    Raicopol, M.D. and Andronescu, C. and Voicu, S.I. and Vasile, E. and Pandele, A.M.
    Carbohydrate Polymers 214 204-212 (2019)
    The increasing amount of residual pharmaceutical contaminants in wastewater has a negative impact on both the environment and human health. In the present study, we developed new cellulose acetate/Mg-Al layered double hydroxide (Mg-Al LDH) nanocomposite membranes as an efficient method to remove pharmaceutical substances from wastewater. The morphology, porosity, surface properties and thermal stability of nanocomposite membranes containing various amounts of nanofiller were evaluated by scanning electron microscopy (SEM), X-ray microtomography (μCT), contact angle measurements and thermogravimetric analysis (TGA). The Mg-Al LDH nanofiller showed a high degree of exfoliation in the polymer matrix, evidenced by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The hydrodynamic properties and adsorption capacity were evaluated with pure water and aqueous solutions of two common drugs, diclofenac sodium (DS) and tetracycline (TC), and the nanocomposite membranes showed an improved permeability compared with neat cellulose acetate. The membrane prepared with 4 wt.% Mg-Al LDH loading exhibited the highest water flux compared with the pure polymer one (529 vs 36 L·m −2 ·h -1 ) and a tenfold increase in adsorption capacity for DS. This enhancement is attributed to electrostatic interactions between the negatively charged drug molecule and positively charged Mg-Al LDH layers. Conversely, in the case of TC, the increase in adsorption capacity was smaller and was assigned to hydrogen bonding interactions between the drug molecule and the nanofiller. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.carbpol.2019.03.042
  • 2019 • 184 Surface modification by polyzwitterions of the sulfabetaine-type, and their resistance to biofouling
    Schönemann, E. and Laschewsky, A. and Wischerhoff, E. and Koc, J. and Rosenhahn, A.
    Polymers 11 (2019)
    Films of zwitterionic polymers are increasingly explored for conferring fouling resistance to materials. Yet, the structural diversity of polyzwitterions is rather limited so far, and clear structure-property relationships are missing. Therefore, we synthesized a series of new polyzwitterions combining ammonium and sulfate groups in their betaine moieties, so-called poly(sulfabetaine)s. Their chemical structures were varied systematically, the monomers carrying methacrylate, methacrylamide, or styrene moieties as polymerizable groups. High molar mass homopolymers were obtained by free radical polymerization. Although their solubilities in most solvents were very low, brine and lower fluorinated alcohols were effective solvents in most cases. A set of sulfabetaine copolymers containing about 1 mol % (based on the repeat units) of reactive benzophenone methacrylate was prepared, spin-coated onto solid substrates, and photo-cured. The resistance of these films against the nonspecific adsorption by two model proteins (bovine serum albumin-BSA, fibrinogen) was explored, and directly compared with a set of references. The various polyzwitterions reduced protein adsorption strongly compared to films of poly(n-butyl methacrylate) that were used as a negative control. The poly(sulfabetaine)s showed generally even somewhat higher anti-fouling activity than their poly(sulfobetaine) analogues, though detailed efficacies depended on the individual polymer-protein pairs. Best samples approach the excellent performance of a poly(oligo(ethylene oxide) methacrylate) reference. © 2019 by the authors.
    view abstractdoi: 10.3390/polym11061014
  • 2018 • 183 Adsorption Thermodynamics and Kinetics of Light Hydrocarbons on Microporous Activated Carbon at Low Temperatures
    Birkmann, F. and Pasel, C. and Luckas, M. and Bathen, D.
    Industrial and Engineering Chemistry Research 57 8023-8035 (2018)
    The removal of light hydrocarbons from exhaust air and process gas is important for a variety of applications, e.g., in natural-gas treatment. However, particularly at lower concentrations, the removal of C1 and C2 hydrocarbons is either very expensive or unfeasible with standard technology. Adsorption processes at temperatures below 0 °C may provide a technical solution, but until today, no systematic study on the dynamics of trace adsorption at low temperatures is available. Therefore, in this work, we present breakthrough curve experiments of ethane, propane, and n-butane over a temperature range from +20 to -80 °C and a concentration range from 5 to 1000 Pa on microporous activated carbon. Equilibrium loadings are calculated and modeled with the temperature-dependent Toth equation. From dynamic simulations of the experimental breakthrough curves, kinetic parameters are determined. The lowering of temperature results in the slowdown of kinetics, which, however, is overcompensated by a simultaneous capacity gain. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.iecr.8b00678
  • 2018 • 182 Human Apolipoprotein A1 at Solid/Liquid and Liquid/Gas Interfaces
    Dogan, S. and Paulus, M. and Forov, Y. and Weis, C. and Kampmann, M. and Cewe, C. and Kiesel, I. and Degen, P. and Salmen, P. and Rehage, H. and Tolan, M.
    Journal of Physical Chemistry B 122 3953-3960 (2018)
    An X-ray reflectivity study on the adsorption behavior of human apolipoprotein A1 (apoA1) at hydrophilic and hydrophobic interfaces is presented. It is shown that the protein interacts via electrostatic and hydrophobic interactions with the interfaces, resulting in the absorption of the protein. pH dependent measurements at the solid/liquid interface between silicon dioxide and aqueous protein solution show that in a small pH range between pH 4 and 6, adsorption is increased due to electrostatic attraction. Here, the native shape of the protein seems to be conserved. In contrast, the adsorption at the liquid/gas interface is mainly driven by hydrophobic effects, presumably by extending the hydrophobic regions of the amphipathic helices, and results in a conformational change of the protein during adsorption. However, the addition of differently charged membrane-forming lipids at the liquid/gas interface illustrates the ability of apoA1 to include lipids, resulting in a depletion of the lipids from the interface. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcb.7b12481
  • 2018 • 181 Streptavidin-coated surfaces suppress bacterial colonization by inhibiting non-specific protein adsorption
    Ettelt, V. and Ekat, K. and Kämmerer, P.W. and Kreikemeyer, B. and Epple, M. and Veith, M.
    Journal of Biomedical Materials Research - Part A 106 758-768 (2018)
    Streptavidin is a 58 kDa tetrameric protein with the highest known affinity to biotin with a wide range of applications in bionanotechnology and molecular biology. Dissolved streptavidin is stable at a broad range of temperature, pH, proteolytic enzymes and exhibits low non-specific binding. In this study, a streptavidin monolayer was assembled directly on a biotinylated TiO2-surface to investigate its stability against proteolytic digestion and its suppression of initial bacterial adsorption of Escherichia coli, Bacillus subtilis, and Streptococcus intermedius. In contrast to nonmodified TiO2 surfaces, streptavidin-coated substrates showed only a negligible non-specific protein adsorption at physiological protein concentrations as well as a significantly reduced bacterial adhesion. The antiadhesive properties were demonstrated to be the main reason for the suppression of bacterial adhesion, which makes this approach a promising option for future surface biofunctionalization applications. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 758–768, 2018. © 2017 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/jbm.a.36276
  • 2018 • 180 Adsorption Behavior of Lysozyme at Titanium Oxide-Water Interfaces
    Forov, Y. and Paulus, M. and Dogan, S. and Salmen, P. and Weis, C. and Gahlmann, T. and Behrendt, A. and Albers, C. and Elbers, M. and Schnettger, W. and Egger, S. and Zwar, E. and Rehage, H. and Kiesel, I. and Riedl, T. and Tolan, M.
    Langmuir 34 5403-5408 (2018)
    We present an in situ X-ray reflectivity study of the adsorption behavior of the protein lysozyme on titanium oxide layers under variation of different thermodynamic parameters, such as temperature, hydrostatic pressure, and pH value. Moreover, by varying the layer thickness of the titanium oxide layer on a silicon wafer, changes in the adsorption behavior of lysozyme were studied. In total, we determined less adsorption on titanium oxide compared with silicon dioxide, while increasing the titanium oxide layer thickness causes stronger adsorption. Furthermore, the variation of temperature from 20 to 80 °C yields an increase in the amount of adsorbed lysozyme at the interface. Additional measurements with variation of the pH value of the system in a region between pH 2 and 12 show that the surface charge of both protein and titanium oxide has a crucial role in the adsorption process. Further pressure-dependent experiments between 50 and 5000 bar show a reduction of the amount of adsorbed lysozyme with increasing pressure. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.8b00280
  • 2018 • 179 Potential-pulse assisted thiol chemisorption minimizes non-specific adsorptions in DNA assays
    Jambrec, D. and Conzuelo, F. and Zhao, B. and Schuhmann, W.
    Electrochimica Acta 276 233-239 (2018)
    Effective suppression of non-specific adsorption is crucial for high-sensitive DNA hybridization sensors. Thiol-based passivation layers as generally used in DNA sensors often exhibit insufficient surface blocking and hence additional blocking agents are used to fully protect the surface from non-specific adsorption. We demonstrate that potential pulse-assisted thiol self-assembly on gold electrodes as surface passivation step in DNA assays provides minimized non-specific adsorption as evidenced by chronoamperometric and SECM measurements. Fast and highly efficient surface blocking is achieved avoiding the necessity of additional blocking steps. © 2018
    view abstractdoi: 10.1016/j.electacta.2018.04.180
  • 2018 • 178 Water Adsorption on Clean and Defective Anatase TiO2 (001) Nanotube Surfaces: A Surface Science Approach
    Kenmoe, S. and Lisovski, O. and Piskunov, S. and Bocharov, D. and Zhukovskii, Y.F. and Spohr, E.
    Journal of Physical Chemistry B 122 5432-5440 (2018)
    We use ab initio molecular dynamics simulations to study the adsorption of thin water films with 1 and 2 ML coverage on anatase TiO2 (001) nanotubes. The nanotubes are modeled as 2D slabs, which consist of partially constrained and partially relaxed structural motifs from nanotubes. The effect of anion doping on the adsorption is investigated by substituting O atoms with N and S impurities on the nanotube slab surface. Due to strain-induced curvature effects, water adsorbs molecularly on defect-free surfaces via weak bonds on Ti sites and H bonds to surface oxygens. While the introduction of an S atom weakens the interaction of the surface with water, which adsorbs molecularly, the presence of an N impurity renders the surface more reactive to water, with a proton transfer from the water film and the formation of an NH group at the N site. At 2 ML coverage, a further surface-assisted proton transfer takes place in the water film, resulting in the formation of an OH- group and an NH2+ cationic site on the surface. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcb.7b11697
  • 2018 • 177 Water Adsorption on Clean and Defective Anatase TiO 2 (001) Nanotube Surfaces: A Surface Science Approach
    Kenmoe, S. and Lisovski, O. and Piskunov, S. and Bocharov, D. and Zhukovskii, Y.F. and Spohr, E.
    Journal of Physical Chemistry B 122 5432-5440 (2018)
    We use ab initio molecular dynamics simulations to study the adsorption of thin water films with 1 and 2 ML coverage on anatase TiO 2 (001) nanotubes. The nanotubes are modeled as 2D slabs, which consist of partially constrained and partially relaxed structural motifs from nanotubes. The effect of anion doping on the adsorption is investigated by substituting O atoms with N and S impurities on the nanotube slab surface. Due to strain-induced curvature effects, water adsorbs molecularly on defect-free surfaces via weak bonds on Ti sites and H bonds to surface oxygens. While the introduction of an S atom weakens the interaction of the surface with water, which adsorbs molecularly, the presence of an N impurity renders the surface more reactive to water, with a proton transfer from the water film and the formation of an NH group at the N site. At 2 ML coverage, a further surface-assisted proton transfer takes place in the water film, resulting in the formation of an OH - group and an NH2 + cationic site on the surface. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcb.7b11697
  • 2018 • 176 Retention mechanisms of 1.7 nm ZnS quantum dots and sub-20 nm Au nanoparticles in ultrafiltration membranes
    Lee, H. and Segets, D. and Süß, S. and Peukert, W. and Chen, S.-C. and Pui, D.Y.H.
    Journal of Membrane Science 567 58-67 (2018)
    Membrane processes are considered to be a very effective and promising method for drinking water and wastewater treatments. However, particle removal mechanisms have not been fully elucidated due to complex surface interactions between colloids and membranes, especially for very small colloidal particles. In this study, a series of systematic filtration tests for eight different types of membrane filters, having nominal pore sizes from 0.005 to 0.1 µm, against 1.7 nm ZnS quantum dots (QDs) and 5, 10 and 20 nm Au nanoparticles (NPs) was performed to understand their retention mechanisms, including rejection in front of the filter surface and adsorption inside the filter. By comparing rejection, adsorption and recovery, it was found that the predominant retention mechanisms for retaining small NPs varied from filter to filter. For instance, electrostatic repulsion played a significant role for the rejection of NPs, i.e. impeding them entering the membrane pores in most membranes. In comparison, the Nylon membrane had a significant adsorption retention ability for Au NPs due to electrostatic attraction. Besides, it was found that filtration flow rate, or flux, was also an important parameter for the final retention because the enhanced hydrodynamic drag could trigger the detachment of deposited NPs or press NPs flowing through the superficial entrance leading to penetration. Tests of 10 nm Au NP retention using five different membranes with the same nominal pore size of 0.1 µm showed large variation of NP retention efficiencies demonstrating that pore size should not be used as the only criterion for rating filter performance, especially for small NPs. Our results provide not only detailed insights into the retention mechanisms of various membranes but also suggestions on how to select membrane filters for different filtration purposes. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2018.09.033
  • 2018 • 175 Efficient adsorption and sustainable degradation of gaseous acetaldehyde and o-xylene using rGO-TiO2 photocatalyst
    Lin, W. and Xie, X. and Wang, X. and Wang, Y. and Segets, D. and Sun, J.
    Chemical Engineering Journal 349 708-718 (2018)
    Two types of volatile organic chemicals (VOCs), acetaldehyde and o-xylene, were selected to probe the different adsorption and photodegradation mechanism of gaseous photocatalysis. Reduced graphene oxide (rGO)-TiO2 nanocomposites were prepared by facile solvothermal process to perform the photocatalytic reactions. In the experiments, the removal efficiencies of the acetaldehyde and o-xylene at 80 mL·min−1 flow rate were only 15% and 12% when P25 was applied, while the efficiencies were sharply increased to 42% and 54% by using 0.5 wt% rGO-TiO2 as the photocatalyst, respectively. Interestingly, it is notable that the removal efficiency of o-xylene was higher than that of acetaldehyde with identical rGO-TiO2 photocatalyst. Experiments suggested that there were possibly two reasons. Firstly, the adsorbance of o-xylene was more than that of acetaldehyde owing to the π-π conjugation between rGO and aromatic compounds, which was proved by adsorption equilibrium and TPD tests. ESR tests proved that rGO can promote the generation of surface [rad]OH radicals and depress the [rad]O2 − radicals formation. Compared with the dominant role played by [rad]O2 − radicals in the degradation of acetaldehyde, an almost equal position of [rad]O2 − and [rad]OH radicals was observed in the degradation of o-xylene according to the subsequent radical scavenger experiments. Moreover, the optimized rGO-TiO2 exhibited sustainable photocatalytic activity at 40–120 mL·min−1 flow rate through 160 min tests, while P25 was deactivate only after 25 min. This work demonstrated the different adsorption and degradation characteristics of two types of VOCs, which could propel the target orientation design of photocatalyst in VOCs removal applications. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.cej.2018.05.107
  • 2018 • 174 Enhanced electronic and magnetic properties by functionalization of monolayer GaS via substitutional doping and adsorption
    Ur Rahman, A. and Rahman, G. and Kratzer, P.
    Journal of Physics Condensed Matter 30 (2018)
    The structural, electronic, and magnetic properties of two-dimensional (2D) GaS are investigated using density functional theory (DFT). After confirming that the pristine 2D GaS is a non-magnetic, indirect band gap semiconductor, we consider N and F as substitutional dopants or adsorbed atoms. Except for N substituting for Ga (NGa), all considered cases are found to possess a magnetic moment. Fluorine, both in its atomic and molecular form, undergoes a highly exothermic reaction with GaS. Its site preference (FS or FGa) as substitutional dopant depends on Ga-rich or S-rich conditions. Both for FGa and F adsorption at the Ga site, a strong F-Ga bond is formed, resulting in broken bonds within the GaS monolayer. As a result, FGa induces p-type conductivity in GaS, whereas FS induces a dispersive, partly occupied impurity band about 0.5 e below the conduction band edge of GaS. Substitutional doping with N at both the S and the Ga site is exothermic when using N atoms, whereas only the more favourable site under the prevailing conditions can be accessed by the less reactive N2 molecules. While NGa induces a deep level occupied by one electron at 0.5 eV above the valence band, non-magnetic NS impurities in sufficiently high concentrations modify the band structure such that a direct transition between N-induced states becomes possible. This effect can be exploited to render monolayer GaS a direct-band gap semiconductor for optoelectronic applications. Moreover, functionalization by N or F adsorption on GaS leads to in-gap states with characteristic transition energies that can be used to tune light absorption and emission. These results suggest that GaS is a good candidate for design and construction of 2D optoelectronic and spintronics devices. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aab8b8
  • 2018 • 173 On-surface nickel porphyrin mimics the reactive center of an enzyme cofactor
    Zamborlini, G. and Jugovac, M. and Cossaro, A. and Verdini, A. and Floreano, L. and Lüftner, D. and Puschnig, P. and Feyer, V. and Schneider, C.M.
    Chemical Communications 54 13423-13426 (2018)
    Metal-containing enzyme cofactors achieve their unusual reactivity by stabilizing uncommon metal oxidation states with structurally complex ligands. In particular, the specific cofactor promoting both methanogenesis and anaerobic methane oxidation is a porphyrinoid chelated to a nickel(i) atom via a multi-step biosynthetic path, where nickel reduction is achieved through extensive molecular hydrogenation. Here, we demonstrate an alternative route to porphyrin reduction by charge transfer from a selected copper substrate to commercially available 5,10,15,20-tetraphenyl-porphyrin nickel(ii). X-ray absorption measurements at the Ni L3-edge unequivocally show that NiTPP species adsorbed on Cu(100) are stabilized in the highly reactive Ni(i) oxidation state by electron transfer to the molecular orbitals. Our approach highlights how some fundamental properties of synthetically inaccessible biological cofactors may be reproduced by hybridization of simple metalloporphyrins with metal surfaces, with implications towards novel approaches to heterogenous catalysis. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8cc06739b
  • 2017 • 172 Adsorption of Cyclic Hydrocarbons on Silica Alumnia Gels in Natural Gas Processing
    Berg, F. and Pasel, C. and Luckas, M. and Eckardt, T. and Bathen, D.
    Chemie-Ingenieur-Technik (2017)
    To ensure the technical use of natural gas it is necessary to remove heavy (cyclic) hydrocarbons from the crude gas. In this article the adsorption of two cyclic hydrocarbons in a N2-carrier gas is investigated on two commercial silica alumina gels. Adsorption isotherms are determined from cumulative breakthrough experiments in a fixed-bed adsorber. The isosteric heat of adsorption is calculated from the temperature-dependent isotherms. The experimental data is discussed regarding the molecular structure of the adsorptive agents as well as surface chemistry and pore structure of the adsorbents. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201600183
  • 2017 • 171 Trace Adsorption of Ethane, Propane, and n-Butane on Microporous Activated Carbon and Zeolite 13X at Low Temperatures
    Birkmann, F. and Pasel, C. and Luckas, M. and Bathen, D.
    Journal of Chemical and Engineering Data 62 1973-1982 (2017)
    Removing of trace-leveled light hydrocarbons from exhaust air or gas streams becomes an increasingly important issue in the field of process and environmental technology, e.g., storage and transport of liquefied natural gas. Adsorption processes at temperatures below 0°C have great potential to meet process specifications or environmental regulatory limits. Designing of such adsorption processes requires a profound insight into the thermodynamics of adsorption at low temperatures, which is not available yet. Therefore, this work provides adsorption isotherms of ethane, propane, and n-butane on microporous activated carbon and zeolite 13X in a temperature range from -40 to +60°C and at partial pressures from 5 to 1000 Pa. The influence of temperature on the adsorbed amount on activated carbon and zeolite 13X is discussed for each adsorptive considering isosteric heats of adsorption and specific interactions between the adsorptive and the adsorbent surface. (Graph Presented). © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jced.6b01068
  • 2017 • 170 Surface-Enhanced Raman Spectroscopy and Density Functional Theory Calculations of a Rationally Designed Rhodamine with Thiol Groups at the Xanthene Ring
    Brem, S. and Schlücker, S.
    Journal of Physical Chemistry C 121 15310-15317 (2017)
    Rhodamines are widely used dyes in fluorescence and surface-enhanced Raman spectroscopy (SERS). The latter requires adsorption of the dye onto the surface of plasmonic nanostructures, a process which requires attractive molecule-surface interactions. Here, we report an experimental SERS and computational density functional theory (DFT) study investigating the role of thiol functionalization at the xanthene ring of the rhodamine in the adsorption onto gold nanoparticles. For this purpose, a new bisthiolated rhodamine derivative was rationally designed and synthesized via a PPh3/I2 reduction route. The introduction of two thiol moieties directly at the xanthene ring provides the shortest possible distance between the molecular π-system and the metal surface for maximum SERS enhancement combined with the strong Au-S interaction for chemisorption. The comparison of experimental SERS spectra obtained from gold nanostars and a film of gold nanoparticles with results from DFT calculations (molecular electrostatic potential, normal modes) suggests adsorption via the thiol groups at the xanthene moiety. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.7b01504
  • 2017 • 169 Modeling of Water Adsorption in SAPO-34-Coated Aluminum Foam
    Bruckner, S. and Demmer, T. and Ganswind, M. and Bathen, D.
    Chemie-Ingenieur-Technik 89 757-764 (2017)
    After presenting the most promising continuous adsorption cooling topology for air-conditioning of a passenger cabin, a transient, 3D model is proposed, based on a modified linear driving force approach, to simulate an adsorption process in a SAPO-34-coated open-celled aluminum foam. The simulation is executed in ANSYS CFX 15.0 and the additional model equations are implemented through the CFX expression language. As a result the influence of the pore diameter onto the system simulation and the mass transport limitation is shown and the sensitivity concerning the pressure in the gas phase adsorption is confirmed. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cite.201600035
  • 2017 • 168 Adsorption of Inhalation Anesthetics (Fluranes and Ethers) on Activated Carbons and Zeolites at Trace Level Concentrations
    Bucher, D. and Pasel, C. and Luckas, M. and Bentgens, J. and Bathen, D.
    Journal of Chemical and Engineering Data 62 1832-1841 (2017)
    This paper presents adsorption isotherms and desorption data of halogenated ethers (desflurane, isoflurane, and sevoflurane), which are used as inhalation anesthetics, and aliphatic ethers (diethyl ether and methyl propyl ether) from a nitrogen carrier gas stream. The isotherms were measured at 25 °C and 1 bar. Desorption was analyzed at temperatures between 25 and 250 °C. Zeolites with different structure and aluminum content as well as activated carbons with different pore size distributions were used as adsorbents. The analysis of the adsorption isotherms and the desorption data focuses on the interactions between adsorptive molecules and adsorbents' surfaces. We discuss the specific influence of the molecular structure and polarity of the adsorptives as well as of the pore structure and surface chemistry of the adsorbents. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.jced.7b00079
  • 2017 • 167 Compositional fingerprint of soy sauces via hydrophobic surface interaction
    Jakobi, V. and Salmen, P. and Paulus, M. and Tolan, M. and Rosenhahn, A.
    Food Chemistry 218 256-260 (2017)
    In this work, the interaction of soy sauces with hydrophobic surfaces has been analyzed. Hydrophobic self-assembled monolayers on gold or silicon dioxide were used to harvest conditioning layers from soy sauce products with varying amounts of additives. The data was compared to adsorption of soy protein and glutamic acid as common ingredients. Spectral ellipsometry revealed that all tested sauces led to the formation of thin overlayers on hydrophobic surfaces. Products with less additives yielded adlayers in the same thickness range as pure soy protein. In contrast, sauces with more ingredients create distinctly thicker films. Using water contact angle goniometry, it is shown that all adlayers render the substrate more hydrophilic. Infrared spectroscopy provided a deeper insight into the adlayer chemistry and revealed that the adlayer composition is dominated by protein rich components. X-ray reflectivity on selected films provided further insight into the density profiles within the adlayers on the molecular scale. © 2016 Elsevier Ltd
    view abstractdoi: 10.1016/j.foodchem.2016.09.045
  • 2017 • 166 Adsorption of finite semiflexible polymers and their loop and tail distributions
    Kampmann, T.A. and Kierfeld, J.
    Journal of Chemical Physics 147 (2017)
    We discuss the adsorption of semiflexible polymers to a planar attractive wall and focus on the questions of the adsorption threshold for polymers of finite length and their loop and tail distributions using both Monte Carlo simulations and analytical arguments. For the adsorption threshold, we find three regimes: (i) a flexible or Gaussian regime if the persistence length is smaller than the adsorption potential range, (ii) a semiflexible regime if the persistence length is larger than the potential range, and (iii) for finite polymers, a novel crossover to a rigid rod regime if the deflection length exceeds the contour length. In the flexible and semiflexible regimes, finite size corrections arise because the correlation length exceeds the contour length. In the rigid rod regime, however, it is essential how the global orientational or translational degrees of freedom are restricted by grafting or confinement. We discuss finite size corrections for polymers grafted to the adsorbing surface and for polymers confined by a second (parallel) hard wall. Based on these results, we obtain a method to analyze adsorption data for finite semiflexible polymers such as filamentous actin. For the loop and tail distributions, we find power laws with an exponential decay on length scales exceeding the correlation length. We derive and confirm the loop and tail power law exponents for flexible and semiflexible polymers. This allows us to explain that, close to the transition, semiflexible polymers have significantly smaller loops and both flexible and semiflexible polymers desorb by expanding their tail length. The tail distribution allows us to extract the free energy per length of adsorption for actin filaments from experimental data [D. Welch et al., Soft Matter 11, 7507 (2015)]. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4990418
  • 2017 • 165 Influence of Tail Groups during Functionalization of ZnO Nanoparticles on Binding Enthalpies and Photoluminescence
    Lin, W. and Schmidt, J. and Mahler, M. and Schindler, T. and Unruh, T. and Meyer, B. and Peukert, W. and Segets, D.
    Langmuir 33 13581-13589 (2017)
    We report on the tailoring of ZnO nanoparticle (NP) surfaces by catechol derivatives (CAT) with different functionalities: tert-butyl group (tertCAT), hydrogen (pyroCAT), aromatic ring (naphCAT), ester group (esterCAT), and nitro group (nitroCAT). The influence of electron-donating/-withdrawing properties on enthalpy of ligand binding (ΔH) was resolved and subsequently linked with optical properties. First, as confirmed by ultraviolet/visible (UV/vis) and Fourier transform infrared (FT-IR) spectroscopy results, all CAT molecules chemisorbed to ZnO NPs, independent of the distinct functionality. Interestingly, the ζ-potentials of ZnO after functionalization shifted to more negative values. Then, isothermal titration calorimetry (ITC) and a mass-based method were applied to resolve the heat release during ligand binding and the adsorption isotherm, respectively. However, both heat- and mass-based approaches alone did not fully resolve the binding enthalpy of each molecule adsorbing to the ZnO surface. This is mainly due to the fact that the Langmuir model oversimplifies the underlying adsorption mechanism, at least for some of the tested CAT molecules. Therefore, a new, fitting-free approach was developed to directly access the adsorption enthalpy per molecule during functionalization by dividing the heat release measured via ITC by the amount of bound molecules determined from the adsorption isotherm. Finally, the efficiency of quenching the visible emission caused by ligand binding was investigated by photoluminescence (PL) spectroscopy, which turned out to follow the same trend as the binding enthalpy. Thus, the functionality of ligand molecules governs the binding enthalpy to the particle surface, which in turn, at least in the current case of ZnO, is an important parameter for the quenching of visible emission. We believe that establishing such correlations is an important step toward a more general way of selecting and designing ligand molecules for surface functionalization. This allows developing strategies for tailored colloidal surfaces beyond empirically driven formulation on a case by case basis. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.7b03079
  • 2017 • 164 The role of the van der Waals interactions in the adsorption of anthracene and pentacene on the Ag(111) surface
    Morbec, J.M. and Kratzer, P.
    Journal of Chemical Physics 146 (2017)
    Using first-principles calculations based on density-functional theory (DFT), we investigated the effects of the van der Waals (vdW) interactions on the structural and electronic properties of anthracene and pentacene adsorbed on the Ag(111) surface. We found that the inclusion of vdW corrections strongly affects the binding of both anthracene/Ag(111) and pentacene/Ag(111), yielding adsorption heights and energies more consistent with the experimental results than standard DFT calculations with generalized gradient approximation (GGA). For anthracene/Ag(111) the effect of the vdW interactions is even more dramatic: we found that “pure” DFT-GGA calculations (without including vdW corrections) result in preference for a tilted configuration, in contrast to the experimental observations of flat-lying adsorption; including vdW corrections, on the other hand, alters the binding geometry of anthracene/Ag(111), favoring the flat configuration. The electronic structure obtained using a self-consistent vdW scheme was found to be nearly indistinguishable from the conventional DFT electronic structure once the correct vdW geometry is employed for these physisorbed systems. Moreover, we show that a vdW correction scheme based on a hybrid functional DFT calculation (HSE) results in an improved description of the highest occupied molecular level of the adsorbed molecules. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4973839
  • 2016 • 163 Development of a Device for Coupling of Calorimetric and Volumetric Sorption Measurements
    Bläker, C. and Luckas, M. and Pasel, C. and Dreisbach, F. and Bathen, D.
    Chemie-Ingenieur-Technik 88 282-290 (2016)
    A new measuring device for the simultaneous measurement of heat of adsorption and load has been developed. A volumetric adsorption measurement device is extended by a calorimetric unit which measures the pressure difference between two identical sensor gas volumes surrounding a sample cell and a reference cell. Due to the exothermic adsorption a pressure rise in the gas volume around the sample cell is induced. After calibration the heat of adsorption can be calculated from the pressure difference curve. First results of the measurement of the adsorption enthalpy of CO2 on 13X zeolite are shown. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201500142
  • 2016 • 162 Adsorption of Helium Atoms on Two-Dimensional Substrates
    Burganova, R. and Lysogorskiy, Y. and Nedopekin, O. and Tayurskii, D.
    Journal of Low Temperature Physics 185 392-398 (2016)
    The study of the adsorption phenomenon of helium began many decades ago with the discovery of graphite as a homogeneous substrate for the investigation of physically adsorbed monolayer films. In particular, helium monoatomic layers on graphite were found to exhibit a very rich phase diagram. In the present work we have investigated the adsorption phenomenon of helium atoms on graphene and silicene substrates by means of density functional theory with Born–Oppenheimer approximation. Helium–substrate and helium–helium interactions were considered from first principles. Vibrational properties of adsorbed monolayers have been used to explore the stability of the system. This approach reproduces results describing the stability of a helium monolayer on graphene calculated by quantum Monte Carlo (QMC) simulations for low and high coverage cases. However, for the moderate coverage value there is a discrepancy with QMC results due to the lack of helium zero point motion. © 2016, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10909-016-1473-4
  • 2016 • 161 Temperature Dependent Adsorption of Sulfur Components, Water, and Carbon Dioxide on a Silica-Alumina Gel Used in Natural Gas Processing
    Chowanietz, V. and Pasel, C. and Luckas, M. and Bathen, D.
    Journal of Chemical and Engineering Data 61 3208-3216 (2016)
    Adsorption is one of the key technologies for the removal of sulfur compounds in trace levels from natural gas prior to a technical utilization. To improve the design of these coupled adsorption-desorption processes a profound insight into the thermodynamics of adsorption is necessary. Therefore, this article provides adsorption isotherms of ethyl mercaptan, methyl mercaptan, hydrogen sulfide, water, and carbon dioxide on a commercial silica-alumina gel used in natural gas purification. The experimental data spans a temperature range between 25 and 300 °C at concentrations between 0 and 2000 mol-ppm at total pressure of 1.3 bar. Equilibrium capacities and isosteric heats of adsorption are compared and discussed based on an analysis of specific interactions between the adsorptives and the adsorbent's chemical surface functionality. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.jced.6b00301
  • 2016 • 160 Modeling and Simulation of a Tube Bundle Adsorber for the Capture of CO2 from Flue Gases
    Duarte, G.S. and Schürer, B. and Voss, C. and Bathen, D.
    Chemie-Ingenieur-Technik 88 336-345 (2016)
    Besides absorption and membrane processes, temperature swing adsorption (TSA) processes allow for the removal of impurities from gas streams and the recovery of the adsorbed component with high purity. However, especially in the case of large quantities of impurities (e.g., CO2 in flue gas) the TSA processes suffer from their high energetic demand. To reduce this energy demand tube bundle adsorbers with indirectly heated and cooled adsorbent bed were developed. The influence of several parameters on the performance of these adsorbers in a capturing process for CO2 from a dry flue gas is investigated. A detailed 2D model is derived and several parameter sweeps are conducted. It is observed that the inner thermal resistance plays a dominant role on the performance of the process. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201500031
  • 2016 • 159 Barrierless growth of precursor-free, ultrafast laser-fragmented noble metal nanoparticles by colloidal atom clusters - A kinetic in situ study
    Jendrzej, S. and Gökce, B. and Amendola, V. and Barcikowski, S.
    Journal of Colloid and Interface Science 463 299-307 (2016)
    Unintended post-synthesis growth of noble metal colloids caused by excess amounts of reactants or highly reactive atom clusters represents a fundamental problem in colloidal chemistry, affecting product stability or purity. Hence, quantified kinetics could allow defining nanoparticle size determination in dependence of the time. Here, we investigate in situ the growth kinetics of ps pulsed laser-fragmented platinum nanoparticles in presence of naked atom clusters in water without any influence of reducing agents or surfactants. The nanoparticle growth is investigated for platinum covering a time scale of minutes to 50 days after nanoparticle generation, it is also supplemented by results obtained from gold and palladium. Since a minimum atom cluster concentration is exceeded, a significant growth is determined by time resolved UV/Vis spectroscopy, analytical disc centrifugation, zeta potential measurement and transmission electron microscopy. We suggest a decrease of atom cluster concentration over time, since nanoparticles grow at the expense of atom clusters. The growth mechanism during early phase (<1. day) of laser-synthesized colloid is kinetically modeled by rapid barrierless coalescence. The prolonged slow nanoparticle growth is kinetically modeled by a combination of coalescence and Lifshitz-Slyozov-Wagner kinetic for Ostwald ripening, validated experimentally by the temperature dependence of Pt nanoparticle size and growth quenching by Iodide anions. © 2015.
    view abstractdoi: 10.1016/j.jcis.2015.10.032
  • 2016 • 158 Physisorption versus chemisorption of oxygen molecules on Ag(100)
    Mehlhorn, M. and Morgenstern, K.
    Journal of Chemical Physics 144 (2016)
    We compare the adsorption of oxygen molecules on Ag(100) at 60 K and at 100 K. At both temperatures, the molecules form islands. Differences between the species adsorbed at the two temperatures in both low-temperature scanning tunneling microscopy and inelastic electron tunneling spectroscopy are attributed to two different adsorption states, a chemisorbed state after 100 K adsorption and a physisorbed state after 60 K adsorption. © 2016 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4945339
  • 2016 • 157 Adsorption and Desorption of Isoflurane on Carbonaceous Adsorbents and Zeolites at Low Concentrations in Gas Phase
    Ortmann, R. and Pasel, C. and Luckas, M. and Heimböckel, R. and Kraas, S. and Bentgens, J. and Fröba, M. and Bathen, D.
    Journal of Chemical and Engineering Data 61 686-692 (2016)
    This paper presents adsorption isotherms and desorption data of isoflurane from a carrier gas (nitrogen) on different adsorbents at 25°C and 1 bar. As adsorbents activated carbons, newly developed carbon adsorbents and dealuminated zeolites were used. The adsorption of isoflurane was studied in trace level concentrations up to 1200 ppmV. Common isotherm equations were fitted to the measured data. The adsorption isotherms show distinctly different capacities depending on the polarity and the pore structure of the adsorbent. The investigation of desorption reveals weak physical interactions between isoflurane and the surface of most adsorbents. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jced.5b00844
  • 2016 • 156 A novel magnetically-separable porous iron-oxide nanocomposite as an adsorbent for methylene blue (MB) dye
    Sehlleier, Y.H. and Hardt, S. and Schulz, C. and Wiggers, H.
    Journal of Environmental Chemical Engineering 4 3779-3787 (2016)
    In this study, efficient and magnetically separable adsorbents for the removal of organic pollutants from water are developed, which are both, environmental friendly and cheap to produce. A new type of porous iron-oxide/polymer nanocomposite was synthesized by a two-step process utilizing surface modification of gas-phase synthesized iron-oxide nanoparticles and a subsequent polymerization process. The potential of iron-oxide/polymer composite adsorbents with a large surface area for the removal of organic components was studied using methylene blue (MB) as a test substance. Adsorption isotherms fitted well with the Langmuir isotherm model and the adsorption capacity of MB on this adsorbent was found to be as high as 298 mg/g which is several times higher than the adsorption capacity of a number of recently reported potential adsorbents. Owing to its magnetic properties, the polluted adsorbent can be easily separated from aqueous solutions. © 2016 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jece.2016.08.018
  • 2016 • 155 Chemical Surface Characterization of Activated Carbons by Adsorption Excess of Probe Molecules
    Treese, J. and Pasel, C. and Luckas, M. and Bathen, D.
    Chemical Engineering and Technology 39 1144-1150 (2016)
    Activated carbons are one of the most common industrial adsorbents in liquid-phase applications. It is known that the surface groups of the activated carbon can have a significant influence on the adsorption process from the liquid phase. Therefore, it is desirable to measure surface groups on activated carbons. This opens up the possibility to use group-contribution methods to model and predict adsorption isotherms. An idea is presented to characterize the inner surface of activated carbons by three types of surface groups: aromatic, polar, and nonpolar surface groups. The amounts of these surface groups were calculated from excess adsorption isotherms of probe molecules on ten activated carbons. This lays the groundwork for further simulation studies of liquid-phase adsorption using group-contribution methods. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/ceat.201500571
  • 2016 • 154 Making the hydrogen evolution reaction in polymer electrolyte membrane electrolysers even faster
    Tymoczko, J. and Calle-Vallejo, F. and Schuhmann, W. and Bandarenka, A.S.
    Nature Communications 7 (2016)
    Although the hydrogen evolution reaction (HER) is one of the fastest electrocatalytic reactions, modern polymer electrolyte membrane (PEM) electrolysers require larger platinum loadings (∼0.5-1.0 mg cm-2) than those in PEM fuel cell anodes and cathodes altogether (∼0.5 mg cm-2). Thus, catalyst optimization would help in substantially reducing the costs for hydrogen production using this technology. Here we show that the activity of platinum(111) electrodes towards HER is significantly enhanced with just monolayer amounts of copper. Positioning copper atoms into the subsurface layer of platinum weakens the surface binding of adsorbed H-intermediates and provides a twofold activity increase, surpassing the highest specific HER activities reported for acidic media under similar conditions, to the best of our knowledge. These improvements are rationalized using a simple model based on structure-sensitive hydrogen adsorption at platinum and copper-modified platinum surfaces. This model also solves a long-lasting puzzle in electrocatalysis, namely why polycrystalline platinum electrodes are more active than platinum(111) for the HER.
    view abstractdoi: 10.1038/ncomms10990
  • 2015 • 153 Detoxification of hexavalent chromium in wastewater containing organic substances using simonkolleite-TiO2 photocatalyst
    Abdel Moniem, S.M. and Ali, M.E.M. and Gad-Allah, T.A. and Khalil, A.S.G. and Ulbricht, M. and El-Shahat, M.F. and Ashmawy, A.M. and Ibrahim, H.S.
    Process Safety and Environmental Protection 95 247-254 (2015)
    Innovative simple method for the preparation of simonkolleite-TiO<inf>2</inf> photocatalyst with different Zn contents was achieved. The prepared photocatalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), FT-IR, Raman and diffuse reflectance spectroscopy techniques. The photocatalytic activities of the materials were evaluated for the simultaneous detoxification of hexavalent chromium (Cr(VI)) and oxidation of organic compounds commonly present in wastewater under simulated solar light. The best photoreduction efficiency of Cr(VI) has been achieved at 1000 ppm simonkolleite-TiO<inf>2</inf> photocatalyst of 5% Zn/TiO<inf>2</inf> weight ratio, and pH value of 2.5 to enhance the adsorption onto catalyst surface. Photoreduction was significantly improved by using formic acid as holes scavenger owing to its chemical adsorption on the catalyst surface. Finally, 100% photoreduction of Cr(VI) could be achieved using formic/simonkolleite-TiO<inf>2</inf> systems under sunlight.
    view abstractdoi: 10.1016/j.psep.2015.03.010
  • 2015 • 152 Characteristics of flexibility in metal-organic framework solid solutions of composition [Zn2(BME-bdc)x(DB-bdc)2-xdabco]n: In situ powder X-ray diffraction, in situ NMR spectroscopy, and molecular dynamics simulations
    Bon, V. and Pallmann, J. and Eisbein, E. and Hoffmann, H.C. and Senkovska, I. and Schwedler, I. and Schneemann, A. and Henke, S. and Wallacher, D. and Fischer, R.A. and Seifert, G. and Brunner, E. and Kaskel, S.
    Microporous and Mesoporous Materials 216 64-74 (2015)
    Porosity switching in the crystalline solid state is a unique phenomenon observed only in a limited number of materials. The switching behavior of two metal-organic frameworks as well as their respective solid solutions of composition [Zn2(BME-bdc)x(DB-bdc)2-xdabco]n (x = 2; 1.5; 1.0; 0.5; 0) is studied in situ during the adsorption of CO2 and Xe using X-ray diffraction and NMR techniques. The diffraction data, measured during the adsorption suggest a direct one-step phase transition (switching) from the narrow pore phase to the large pore phase beyond the transition pressure. An intermediate phase was found only in one compound within a narrow pressure range around the phase transition pressure region. In situ high-pressure 13C NMR spectroscopy of adsorbed CO2 also allowed following the gating behavior of the studied materials by monitoring the signal of adsorbed CO2. The 13C NMR spectra exhibit a pronounced broadening indicating a certain degree of order for the adsorbed molecules inside the pores. This ordering effect and the resulting line broadening depend on the linker functionalization as could be confirmed by corresponding molecular dynamics (MD) simulations. © 2015 Elsevier Inc.
    view abstractdoi: 10.1016/j.micromeso.2015.02.042
  • 2015 • 151 Process boundaries of irreversible scCO2-assisted phase separation in biphasic whole-cell biocatalysis
    Brandenbusch, C. and Glonke, S. and Collins, J. and Hoffrogge, R. and Grunwald, K. and Bühler, B. and Schmid, A. and Sadowski, G.
    Biotechnology and Bioengineering 112 2316-2323 (2015)
    The formation of stable emulsions in biphasic biotransformations catalyzed by microbial cells turned out to be a major hurdle for industrial implementation. Recently, a cost-effective and efficient downstream processing approach, using supercritical carbon dioxide (scCO2) for both irreversible emulsion destabilization (enabling complete phase separation within minutes of emulsion treatment) and product purification via extraction has been proposed by Brandenbusch et al. (2010). One of the key factors for a further development and scale-up of the approach is the understanding of the mechanism underlying scCO2-assisted phase separation. A systematic approach was applied within this work to investigate the various factors influencing phase separation during scCO2 treatment (that is pressure, exposure of the cells to CO2, and changes of cell surface properties). It was shown that cell toxification and cell disrupture are not responsible for emulsion destabilization. Proteins from the aqueous phase partially adsorb to cells present at the aqueous-organic interface, causing hydrophobic cell surface characteristics, and thus contribute to emulsion stabilization. By investigating the change in cell-surface hydrophobicity of these cells during CO2 treatment, it was found that a combination of catastrophic phase inversion and desorption of proteins from the cell surface is responsible for irreversible scCO2 mediated phase separation. These findings are essential for the definition of process windows for scCO2-assisted phase separation in biphasic whole-cell biocatalysis. © 2015 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/bit.25655
  • 2015 • 150 Experimental and theoretical study on the adsorptive drying of primary alcohols in a fixed bed adsorber
    Burrichter, B. and Pasel, C. and Luckas, M. and Bathen, D.
    Separation and Purification Technology 145 39-49 (2015)
    In this paper systematic investigations on the adsorptive drying of primary alcohols from methanol to 1-hexanol on 3A and 4A zeolites are presented. The focus of the work is laid on the superdrying of these polar solvents in the ppmw-region. In addition to equilibrium data gained in shaker bottle experiments information regarding the dynamic drying behavior was obtained by breakthrough experiments. The water content in these experiments was monitored online by a transmission photometer. Based on the experimental data a physical model was developed in order to predict the dynamic drying behavior in a fixed bed adsorber. Most of the investigated systems could be well described by this model. Furthermore pore diffusivities of water could be obtained by fitting the model to the experimental data. The results of this study reveal a clear dependency of the dynamic drying behavior of primary alcohols from the solvent to be dried and the type of zeolite. Small alcohol molecules show significantly faster drying rates than long-chain alcohols and the drying kinetics on a 3A zeolite is much faster than on a 4A zeolite. The experimental results were correlated to the molecular dimensions of adsorptives and the size of the pore openings of the zeolites as well as to the interactions between adsorptive, solvent and adsorbent. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.seppur.2015.02.033
  • 2015 • 149 Finding optimal surface sites on heterogeneous catalysts by counting nearest neighbors
    Calle-Vallejo, F. and Tymoczko, J. and Colic, V. and Vu, Q.H. and Pohl, M.D. and Morgenstern, K. and Loffreda, D. and Sautet, P. and Schuhmann, W. and Bandarenka, A.S.
    Science 350 185-189 (2015)
    A good heterogeneous catalyst for a given chemical reaction very often has only one specific type of surface site that is catalytically active. Widespread methodologies such as Sabatier-type activity plots determine optimal adsorption energies to maximize catalytic activity, but these are difficult to use as guidelines to devise new catalysts. We introduce "coordination-activity plots" that predict the geometric structure of optimal active sites. The method is illustrated on the oxygen reduction reaction catalyzed by platinum. Sites with the same number of first-nearest neighbors as (111) terraces but with an increased number of second-nearest neighbors are predicted to have superior catalytic activity. We used this rationale to create highly active sites on platinum (111), without alloying and using three different affordable experimental methods.
    view abstractdoi: 10.1126/science.aab3501
  • 2015 • 148 The dynamic influence of cells on the formation of stable emulsions in organic–aqueous biotransformations
    Collins, J. and Grund, M. and Brandenbusch, C. and Sadowski, G. and Schmid, A. and Bühler, B.
    Journal of Industrial Microbiology and Biotechnology 42 1011-1026 (2015)
    Emulsion stability plays a crucial role for mass transfer and downstream processing in organic–aqueous bioprocesses based on whole microbial cells. In this study, emulsion stability dynamics and the factors determining them during two-liquid phase biotransformation were investigated for stereoselective styrene epoxidation catalyzed by recombinant Escherichia coli. Upon organic phase addition, emulsion stability rapidly increased correlating with a loss of solubilized protein from the aqueous cultivation broth and the emergence of a hydrophobic cell fraction associated with the organic–aqueous interface. A novel phase inversion-based method was developed to isolate and analyze cellular material from the interface. In cell-free experiments, a similar loss of aqueous protein did not correlate with high emulsion stability, indicating that the observed particle-based emulsions arise from a convergence of factors related to cell density, protein adsorption, and bioreactor conditions. During styrene epoxidation, emulsion destabilization occurred correlating with product-induced cell toxification. For biphasic whole-cell biotransformations, this study indicates that control of aqueous protein concentrations and selective toxification of cells enables emulsion destabilization and emphasizes that biological factors and related dynamics must be considered in the design and modeling of respective upstream and especially downstream processes. © 2015, Society for Industrial Microbiology and Biotechnology.
    view abstractdoi: 10.1007/s10295-015-1621-x
  • 2015 • 147 Novel functionalization of porous polypropylene microfiltration membranes: Via grafted poly(aminoethyl methacrylate) anchored Schiff bases toward membrane adsorbers for metal ions
    Farjadian, F. and Schwark, S. and Ulbricht, M.
    Polymer Chemistry 6 1584-1593 (2015)
    In this work we introduce novel synthetic methods for the modification of a macroporous polypropylene (PP) membrane with poly(2-aminoethyl methacrylate) (polyAEMA) and subsequently anchoring a Schiff base with the aim of adsorbing specific metal ions from aqueous solution. The Schiff base synthesis on the surface of the PP membrane was done by a sequence of reactions. First the hydrophobic character of a commercial PP membrane (pore diameter 0.4 μm) was modified via UV irradiation-induced "grafting-from" of poly(2-hydroxyethyl methacrylate) (polyHEMA). The hydroxyl groups of polyHEMA were then reacted with the pre-synthesized photoinitiator 4-ethoxy-5-oxo-4,5-diphenylpentanoyl bromide. UV-irradiation was thereafter used for the "grafting-from" of polyAEMA. The free amino groups of this grafted comb-like brush layer on the surface were reacted with salicylaldehyde to form a Schiff base. ATR-FTIR spectroscopy was carried out to determine the functional groups' introduction and conversion. Scanning electron microscopy images showed the changes between unmodified and modified membranes. The specific surface area was determined by nitrogen adsorption and BET analysis, and the water permeability was also measured. The efficiency of membrane adsorbers with a Schiff base in the grafted layer in binding to Cu(ii) ions was determined by atomic absorption spectroscopy. Overall, the established functionalization sequences and the obtained functionality have potential for the development of efficient adsorber materials. This journal is © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c4py01521e
  • 2015 • 146 The geometric and electronic structure of TCNQ and TCNQ+Mn on Ag(0 0 1) and Cu(0 0 1) surfaces
    Feyer, V. and Graus, M. and Nigge, P. and Zamborlini, G. and Acres, R.G. and Schöll, A. and Reinert, F. and Schneider, C.M.
    Journal of Electron Spectroscopy and Related Phenomena 204 125-131 (2015)
    Copper and silver surfaces can be used as model systems to study structure formation and interfacial bonding upon adsorption of organic molecules. We have investigated the geometric and electronic structure of ordered monolayers of TCNQ on Cu(0 0 1) and Ag(0 0 1) and of TCNQ+Mn on Ag(0 0 1) surfaces by LEED and photoelectron momentum microscopy. While TCNQ forms an incommensurable superstructure on Cu(0 0 1), two coverage-dependant, commensurable superstructures are established on Ag(0 0 1). Subsequent adsorption of Mn on top of TCNQ/Ag(0 0 1) results in the formation of a long-range ordered mixed metal-organic superstructure, which is also commensurable with the Ag(0 0 1) substrate. The photoelectron spectroscopy (PES) data shows a filling of the TCNQ LUMO by charge transfer from the substrate for all investigated interfaces and the coadsorption of Mn leads to an energy shift of the TCNQ HOMO and LUMO of 230 meV with respect to TCNQ/Ag(0 0 1). The characteristic angle-dependent intensity pattern of the TCNQ LUMO in PES was utilized to investigate the azimuthal orientation of the molecules in the respective unit cells. The angle-resolved PES data was further analyzed to identify lateral band dispersion effects in the adsorbate layers, but no significant dispersion was observed. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elspec.2015.02.010
  • 2015 • 145 Salt induced reduction of lysozyme adsorption at charged interfaces
    Göhring, H. and Paulus, M. and Salmen, P. and Wirkert, F. and Kruse, T. and Degen, P. and Stuhr, S. and Rehage, H. and Tolan, M.
    Journal of Physics Condensed Matter 27 (2015)
    A study of lysozyme adsorption below a behenic acid membrane and at the solid-liquid interface between aqueous lysozyme solution and a silicon wafer in the presence of sodium chloride is presented. The salt concentration was varied between 1 mmol L-1 and 1000 mmol L-1. X-ray reflectivity data show a clear dependence of the protein adsorption on the salt concentration. Increasing salt concentrations result in a decreased protein adsorption at the interface until a complete suppression at high concentrations is reached. This effect can be attributed to a reduced attractive electrostatic interaction between the positively charged proteins and negatively charged surfaces by charge screening. The measurements at the solid-liquid interfaces show a transition from unoriented order of lysozyme in the adsorbed film to an oriented order with the short protein axis perpendicular to the solid-liquid interface with rising salt concentration. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/27/23/235103
  • 2015 • 144 Influence of Adsorption Kinetics upon the Electrochemically Reversible Hydrogen Oxidation Reaction
    Lin, C. and Jiao, X. and Tschulik, K. and Batchelor-Mcauley, C. and Compton, R.G.
    Journal of Physical Chemistry C 119 16121-16130 (2015)
    The hydrogen oxidation reaction was studied at bright polycrystalline platinum microelectrodes. A smaller steady-state current was observed in experiment as compared to that anticipated for a diffusion limited process. To facilitate physical insight into this system, a simulation model based on the Tafel-Volmer mechanism for the hydrogen oxidation reaction was developed. Under conditions of reversible electron transfer, the adsorption kinetics k<inf>a</inf> and k<inf>d</inf> are found to have distinctly different influences upon the voltammetry responses. Correspondence between the simulated and the experimental voltammograms is found, confirming the decrease of the steady-state current is caused by the slow adsorption process. The combined adsorption parameter k<inf>a</inf>γmax2 on the Tafel-Volmer mechanism was approximately 5.0 × 10-4 m s-1, where γ <inf>max</inf> (mol m-2) is the maximum surface coverage of adsorption hydrogen atoms. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b04293
  • 2015 • 143 A general approach to study the thermodynamics of ligand adsorption to colloidal surfaces demonstrated by means of catechols binding to zinc oxide quantum dots
    Lin, W. and Walter, J. and Burger, A. and Maid, H. and Hirsch, A. and Peukert, W. and Segets, D.
    Chemistry of Materials 27 358-369 (2015)
    A general strategy to study the thermodynamics of ligand adsorption to colloidal surfaces was established. The versatility of our approach is demonstrated by means of catechols binding to ZnO quantum dots (QDs). First, isothermal titration calorimetry (ITC) was used to extract all relevant thermodynamic parameters, namely association constant, enthalpy, entropy, and free energy of ligand binding. Noteworthy, the determined δG of -20.3 ± 0.4 kJ mol-1 indicates a strong, reproducible, and exothermic interaction between the catechol anchor group and the oxide particle surface. To confirm the characterization of ligand binding by measuring the heat of adsorption, the free energy was cross-validated by mass-based adsorption isotherms. A combination of inductively coupled plasma optical emission spectroscopy (ICP-OES) and UV/vis spectroscopy was developed to quantitatively determine the mass of bound catechols with respect to the available particle surface. The association constant K was determined by a Langmuir fit to be 2618 M-1 which leads to δG = -19.50 kJ mol-1 according to δG = -RTln K. To close the mass balance, analytical ultracentrifugation (AUC) was applied to detect the amount of the free, unbound catechol in solution. Finally, Raman spectroscopy and nuclear magnetic resonance spectroscopy (NMR) were performed to quantify the amount of remaining acetate from particle synthesis and to distinguish bound (chemisorbed) and unbound (physisorbed) catechol. Our results reveal that approximately 65 wt % of acetate is replaced, and physisorbed catechol will not affect the amount of remaining acetate on the ZnO surface. Moreover, no pronounced chemical shift peak as it would be expected for free catechol is observed by NMR at all. This indicates a highly dynamic adsorption-desorption equilibrium between the free and the physisorbed state of catechol on the particle surface. Our concept of combined analytics is seen to be a generally applicable strategy for particle-ligand interfacial studies. It gives detailed insight into thermodynamics, binding states, and ligand composition and is thus considered as an important step toward tailored colloidal surface properties. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/cm504080d
  • 2015 • 142 Polyvinylamine modified polyester fibers-innovative textiles for the removal of chromate from contaminated groundwater
    Mayer-Gall, T. and Opwis, K. and Gutmann, J.S.
    Journal of Materials Chemistry A 3 386-394 (2015)
    The soil and groundwater of many old industrial areas are polluted by different environmental hazards. Because of their high toxicity and carcinogenic potential, chromate contaminations are especially problematic and a complete cleanup of such areas is necessary to avoid fatal environmental and sanitary impacts. Conventionally, decontamination is carried out by the removal of the soil and a long-term filtration of groundwater with various chromate-adsorbing materials over a period of many years. Efficient, reusable and cheap adsorbing materials, however, are still missing. Here, we present a new, cheap and reusable chromate-adsorbing substrate based on polyvinylamine-coated polyester fibers. The surface modification of the fibrous material is realized by common methods in textile finishing yielding a durable, high-performing and reusable adsorbent for water-dissolved chromate. The functionalized nonwoven fabric has a high binding capacity for chromate and the chromate concentration of highly polluted waters (with concentrations around 50 mg L-1) can easily be decreased below the limit of 50 μg L-1 recommended by the WHO. Moreover, the material is reusable after regeneration under aqueous alkaline conditions. The adsorption properties at different pH values were determined with different adsorption models. In addition, adsorption kinetics were evaluated using artificial and real life chromate-contaminated water samples. This journal is © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c4ta04212c
  • 2015 • 141 Mobility of coated and uncoated TiO2 nanomaterials in soil columns - Applicability of the tests methods of OECD TG 312 and 106 for nanomaterials
    Nickel, C. and Gabsch, S. and Hellack, B. and Nogowski, A. and Babick, F. and Stintz, M. and Kuhlbusch, T.A.J.
    Journal of Environmental Management 157 230-237 (2015)
    Nanomaterials are commonly used in everyday life products and during their life cycle they can be released into the environment. Soils and sediments are estimated as significant sinks for those nanomaterials. To investigate and assess the behaviour of nanomaterials in soils and sediments standardized test methods are needed. In this study the applicability of two existing international standardized test guidelines for the testing of nanomaterials, OECD TG 106 "Adsorption/Desorption using a Bath Equilibrium Method" and the OECD TG 312 "Leaching in Soil Columns", were investigated. For the study one coated and two uncoated TiO<inf>2</inf> nanomaterials were used, respectively. The results indicate that the OECD TG 106 is not applicable for nanomaterials. However, the test method according to OECD TG 312 was found to be applicable if nano-specific adaptations are applied. The mobility investigations of the OECD TG 312 indicated a material-dependent mobility of the nanomaterials, which in some cases may lead to an accumulation in the upper soil layers. Whereas no significant transport was observed for the uncoated materials for the double-coated material (coating with dimethicone and aluminiumoxide) a significant transport was detected and attributed to the coating. © 2015 Elsevier Ltd.
    view abstractdoi: 10.1016/j.jenvman.2015.04.029
  • 2015 • 140 Sorption of Acetaldehyde and Hexanal in Trace Concentrations on Carbon-Based Adsorbents
    Ortmann, R. and Pasel, C. and Luckas, M. and Kraas, S. and Fröba, M. and Bathen, D.
    Chemical Engineering and Technology 38 125-130 (2015)
    Adsorption processes are frequently applied to separate traces of hazardous and toxic substances from gas streams. Hence, knowledge of sorption characteristics of these substances on standard adsorbents is essential. Sorption of hexanal and acetaldehyde from a nitrogen gas stream in trace concentrations on activated carbon and ordered mesoporous carbon-based adsorbents (CMK) is studied. A magnetic suspension balance and an attached gaschromatograph-mass spectrometer were used to analyze the sorption process both gravimetrically and spectrometrically. Both types of adsorbents show a higher capacity for hexanal than acetaldehyde. The activated carbon exhibits considerable differences in regard to desorption of hexanal compared to the mesoporous CMK. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ceat.201400510
  • 2015 • 139 Influence of Solvent-Like Sidechains on the Adsorption of Light Hydrocarbons in Metal-Organic Frameworks
    Schneemann, A. and Bloch, E.D. and Henke, S. and Llewellyn, P.L. and Long, J.R. and Fischer, R.A.
    Chemistry - A European Journal 21 18764-18769 (2015)
    A variety of strategies have been developed to adsorb and separate light hydrocarbons in metal-organic frameworks. Here, we present a new approach in which the pores of a framework are lined with four different C3 sidechains that feature various degrees of branching and saturation. These pendant groups, which essentially mimic a low-density solvent with restricted degrees of freedom, offer tunable control of dispersive host-guest interactions. The performance of a series of frameworks of the type Zn2(fu-bdc)2(dabco) (fu-bdc2-=functionalized 1,4-benzenedicarboxylate; dabco=1,4-diazabicyclo[2.2.2]octane), which feature a pillared layer structure, were investigated for the adsorption and separation of methane, ethane, ethylene, and acetylene. The four frameworks exhibit low methane uptake, whereas C2 hydrocarbon uptake is substantially higher as a result of the enhanced interaction of these molecules with the ligand sidechains. Most significantly, the adsorption quantities and selectivity were found to depend strongly upon the type of sidechains attached to the framework scaffold. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201503685
  • 2015 • 138 Laser-Assisted Wet-Chemical Doping of Sintered Si and Ge Nanoparticle Films
    Stoib, B. and Greppmair, A. and Petermann, N. and Wiggers, H. and Stutzmann, M. and Brandt, M.S.
    Advanced Electronic Materials 1 (2015)
    Doped thin films of group-IV semiconductors can be fabricated using the adsorption of dopant species from a liquid source to a precursor nanoparticle film, followed by laser-sintering to incorporate and activate the dopants in the sintered thin film. A detailed study of the doping of germanium films with arsenic reveals diffusion of dopants into the film and their adsorption to the nanoparticle surface as kinetically governing steps, benefiting from the large internal surface area of the nanoparticle film. The resulting charge carrier concentration can be adjusted by the internal surface area via the nano­particle diameter, by controlling the dopant concentration in the liquid, and by the immersion time and temperature. It is shown that the method can be successfully transferred to silicon and silicon–germanium alloy films using group-III and -V elements, which lead to p- and n-type conductivity, respectively. Atomic dopant concentrations above 1020 cm−3 can be realized by laser-sintering, which are electrically active to a high extent and lead to effective conductivities well above 10 S cm–1 in the mesoporous films is investigated here. The method allows flexible printing of devices using inks for the nanoparticles and the dopant and avoids toxic substances for the doping of nanoparticles in the gas phase. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/aelm.201400029
  • 2015 • 137 A novel approach for the characterization of liquid phase adsorption on activated carbons
    Treese, J. and Pasel, C. and Luckas, M. and Bathen, D.
    Chemie-Ingenieur-Technik 87 563-570 (2015)
    Abstract Activated carbons are widely applicable in industrial adsorption processes. However, characterization of their surface properties is problematic due to the number of different source materials and variations in production processes. Here, a model of the carbon surface derived that is based on the knowledge of fundamental molecular interactions on carbon surfaces. Adsorption isotherms of selected probe molecules are measured on different activated carbons and discussed how the model can be applied in the characterization of the surface properties of activated carbons. Copyright © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201400085
  • 2014 • 136 Influence of zwitterionic SAMs on protein adsorption and the attachment of algal cells
    Bauer, S. and Alles, M. and Finlay, J.A. and Callow, J.A. and Callow, M.E. and Rosenhahn, A.
    Journal of Biomaterials Science, Polymer Edition 25 1530-1539 (2014)
    Zwitterionic polymers are non-fouling materials with immense potential for a range of biological applications. Here, we describe the resistance of zwitterionic self-assembled monolayers prepared from different solution ratios of positively and negatively charged thiols towards the adhesion of proteins, zoospores of the green alga Ulva linza, and cells of the unicellular alga Navicula perminuta. While mixed zwitterionic surfaces with a high hydrophilic nature significantly reduced the adhesion strength of the two algae, the positively and negatively charged components were far less effective. © 2014 Taylor and Francis.
    view abstractdoi: 10.1080/09205063.2014.929429
  • 2014 • 135 Iron oxide/polymer-based nanocomposite material for hydrogen sulfide adsorption applications
    Blatt, O. and Helmich, M. and Steuten, B. and Hardt, S. and Bathen, D. and Wiggers, H.
    Chemical Engineering and Technology 37 1938-1944 (2014)
    The processing of iron oxide nanoparticles derived from spray flame synthesis for specific adsorption applications is described. After the as-prepared particles proved the ability for H2S removal in pure gas treatment, two different nanoparticle- based composite materials were prepared. While impregnation of activated carbon with the as-prepared nanoparticles showed the expected increase in H2S adsorption capacities, a significant enhancement in desulfurization performance was observed for a novel iron oxide nanoparticle composite material. H2S adsorption was tested in fixed-bed breakthrough curve measurements. The H2S removal efficiency of the novel material under ambient conditions indicates highly promising properties for potential use in industrial and air pollution control applications. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ceat.201400303
  • 2014 • 134 Parameter study on the adsorptive drying of isopropanol in a fixed bed adsorber
    Burrichter, B. and Pasel, C. and Luckas, M. and Bathen, D.
    Separation and Purification Technology 132 736-743 (2014)
    This work focuses on the influence of process parameters on the dynamics of adsorptive water removal from polar organic solvents in a fixed bed adsorber. As a model solvent isopropanol with water concentrations between 5 and 4000 ppmw was used. In a first step equilibrium loadings on 3A and 4A zeolites were determined by shaker bottle experiments. The results were fitted to the Langmuir equation. In a second step fixed bed experiments were carried out in order to characterize the dynamic behavior of the adsorption process. In these experiments 3A zeolite shows a better drying performance than 4A zeolite. The breakthrough curves (BTC) could be well described by dynamic simulations using a set of differential equations for the mass balances and a linear driving force approach (LDF) for the kinetics. Pore diffusivities in the order of 10-12 m2/s were obtained by the simulation, indicating that surface diffusion in the pores of the zeolites is the predominant mechanism of mass transfer. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.seppur.2014.06.030
  • 2014 • 133 Effect of preparation of iron-infiltrated activated carbon catalysts on nitrogen oxide conversion at low temperature
    Busch, M. and Schmidt, W. and Migunov, V. and Beckel, A. and Notthoff, C. and Kompch, A. and Bergmann, U. and Winterer, M. and Atakan, B.
    Applied Catalysis B: Environmental 160-161 641-650 (2014)
    Nitrogen oxides are toxic and their concentration in human workspace should be reduced to a minimum level. Among the possible catalyst materials activated carbon based catalysts are a cheap and non-toxic alternative of high availability. In this paper we investigate two different methods for the preparation of iron-infiltrated activated carbon catalysts: chemical vapor infiltration (CVI) and the incipient wetness method (IWM). The effects of the preparation method on the structure and catalytical performance are compared with the effects of infiltration load and co-deposition of silicon dioxide. The study elucidates profound differences in the nitrogen dioxide adsorption and catalytic nitrogen oxide decomposition, depending on the catalyst preparation technique. Samples prepared by chemical vapor infiltration exhibit well dispersed iron/iron oxide particles all over the sample cross section. Crystalline iron oxide is only detected in the samples prepared via the gas phase and not in samples prepared by IWM. The nitrogen dioxide adsorption is notably enhanced in samples with a large accessible micropore volume. All samples containing iron catalyze the conversion of nitrogen oxides into nitrous oxide and carbon monoxide, but especially the co-deposition of silica enhances the nitric oxide conversion into less harmful species. The iron/silica-co-deposited activated carbon catalyst prepared via incipient wetness method exhibits the best catalytical performance of all investigated catalysts at 425. K. © 2014 .
    view abstractdoi: 10.1016/j.apcatb.2014.05.010
  • 2014 • 132 Negatively charged ions on Mg(0001) surfaces: Appearance and origin of attractive adsorbate-adsorbate interactions
    Cheng, S.-T. and Todorova, M. and Freysoldt, C. and Neugebauer, J.
    Physical Review Letters 113 (2014)
    Adsorption of electronegative elements on a metal surface usually leads to an increase in the work function and decrease in the binding energy as the adsorbate coverage rises. Using density-functional theory calculations, we show that Cl adsorbed on a Mg(0001) surface complies with these expectations, but adsorption of {N,O,F} causes a decrease in the work function and an increase in the binding energy. Analyzing the electronic structure, we show that the presence of a highly polarizable electron spill-out in front of Mg(0001) causes this unusual adsorption behavior and is responsible for the appearance of a hitherto unknown net-attractive lateral electrostatic interaction between same charged adsorbates. © 2014 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.113.136102
  • 2014 • 131 Fate and Bioavailability of Engineered Nanoparticles in Soils: A Review,Interactions within natural soils have often been neglectedwhen assessing fate and bioavailability of engineered nanomaterials (ENM) in soils. This review combines patchwise ENM research using natural soils with the much wider literature on ENM performed in standard tests or on the fate of colloids in soils, and an analysis of the diverse ENM characteristics determining availability from the soil organisms' perspective to assess the main soil characteristics that determine the fate, speciation, and ultimately bioavailability of ENM in natural soils. Predominantly salinity, texture, pH, concentration, and nature of mobile organic compounds and degree of saturation determine ENM bioavailability.
    Cornelis, G. and Hund-Rinke, K. and Kuhlbusch, T. and Van Den Brink, N. and Nickel, C.
    Critical Reviews in Environmental Science and Technology 44 2720-2764 (2014)
    Interactions within natural soils have often been neglectedwhen assessing fate and bioavailability of engineered nanomaterials (ENM) in soils. This review combines patchwise ENM research using natural soils with the much wider literature on ENM performed in standard tests or on the fate of colloids in soils, and an analysis of the diverse ENM characteristics determining availability from the soil organisms' perspective to assess the main soil characteristics that determine the fate, speciation, and ultimately bioavailability of ENM in natural soils. Predominantly salinity, texture, pH, concentration, and nature of mobile organic compounds and degree of saturation determine ENM bioavailability.
    view abstractdoi: 10.1080/10643389.2013.829767
  • 2014 • 130 Carbon monoxide-assisted size confinement of bimetallic alloy nanoparticles
    Cui, C. and Gan, L. and Neumann, M. and Heggen, M. and Roldan Cuenya, B. and Strasser, P.
    Journal of the American Chemical Society 136 4813-4816 (2014)
    Colloid-based chemical synthesis methods of bimetallic alloy nanoparticles (NPs) provide good monodispersity, yet generally show a strong variation of the resulting mean particle size with alloy composition. This severely compromises accurate correlation between composition of alloy particles and their size-dependent properties. To address this issue, a general CO adsorption-assisted capping ligand-free solvothermal synthesis method is reported which provides homogeneous bimetallic NPs with almost perfectly constant particle size over an unusually wide compositional range. Using Pt-Ni alloy NPs as an example, we show that variation of the reaction temperature between 160 and 240 °C allows for precise control of the resulting alloy particle bulk composition between 15 and 70 atomic % Ni, coupled with a constant mean particle size of ∼4 nm. The size-confining and Ni content-controlling role of CO during the nucleation and growth processes are investigated and discussed. Data suggest that size-dependent CO surface chemisorption and reversible Ni-carbonyl formation are key factors for the achievement of a constant particle size and temperature-controlled Ni content. To demonstrate the usefulness of the independent control of size and composition, size-deconvoluted relations between composition and electrocatalytic properties are established. Refining earlier reports, we uncover intrinsic monotonic relations between catalytic activity and initial Ni content, as expected from theoretical considerations. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja4124658
  • 2014 • 129 Reflectance difference spectroscopy of water on Cu(110)
    Denk, M. and Hohage, M. and Suna, L.D. and Zeppenfeld, P. and Esser, N. and Cobet, C.
    Surface Science 627 16-22 (2014)
    The adsorption of H2O on Cu(110)was probed bymeans of reflectance difference spectroscopy (RDS) in the energy range between 1.5 and 9.3 eV and by scanning tunnelingmicroscopy (STM). The adsorption ofwater on the pristine Cu(110) substrate mainly induces changes in the Cu surface state related optical transitions. Furthermore, the H2O adsorbate modifies the Cu bulk transitions. In particular, our investigations demonstrate that the coverage-dependent phase transition from 1D pentamer chains to a (7 × 8) superstructure can bemonitored by means of RDS. In the vacuum-UV range, new RD features assigned to Cu bulk transitions were detected. Adsorption on themetal surface stronglymodifies or quenches theH2OHOMO-LUMOtransition, whereby a distinct RD feature of the water molecules themselves in the vacuum-UV range is absent. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2014.04.009
  • 2014 • 128 Structural complexity in metal-organic frameworks: Simultaneous modification of open metal sites and hierarchical porosity by systematic doping with defective linkers
    Fang, Z. and Dürholt, J.P. and Kauer, M. and Zhang, W. and Lochenie, C. and Jee, B. and Albada, B. and Metzler-Nolte, N. and Pöppl, A. and Weber, B. and Muhler, M. and Wang, Y. and Schmid, R. and Fischer, R.A.
    Journal of the American Chemical Society 136 9627-9636 (2014)
    A series of defect-engineered metal-organic frameworks (DEMOFs) derived from parent microporous MOFs was obtained by systematic doping with defective linkers during synthesis, leading to the simultaneous and controllable modification of coordinatively unsaturated metal sites (CUS) and introduction of functionalized mesopores. These materials were investigated via temperature-dependent adsorption/desorption of CO monitored by FTIR spectroscopy under ultra-high-vacuum conditions. Accurate structural models for the generated point defects at CUS were deduced by matching experimental data with theoretical simulation. The results reveal multivariate diversity of electronic and steric properties at CUS, demonstrating the MOF defect structure modulation at two length scales in a single step to overcome restricted active site specificity and confined coordination space at CUS. Moreover, the DEMOFs exhibit promising modified physical properties, including band gap, magnetism, and porosity, with hierarchical micro/mesopore structures correlated with the nature and the degree of defective linker incorporation into the framework. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja503218j
  • 2014 • 127 Adsorption geometry and electronic structure of iron phthalocyanine on Ag surfaces: A LEED and photoelectron momentum mapping study
    Feyer, V. and Graus, M. and Nigge, P. and Wießner, M. and Acres, R.G. and Wiemann, C. and Schneider, C.M. and Schöll, A. and Reinert, F.
    Surface Science 621 64-68 (2014)
    We present a comprehensive study of the adsorption behavior of iron phthalocyanine on the low-index crystal faces of silver. By combining measurements of the reciprocal space by means of photoelectron momentum mapping and low energy electron diffraction, the real space adsorption geometries are reconstructed. At monolayer coverage ordered superstructures exist on all studied surfaces containing one molecule in the unit cell in case of Ag(100) and Ag(111), and two molecules per unit cell for Ag(110). The azimuthal tilt angle of the molecules against the high symmetry directions of the substrate is derived from the photoelectron momentum maps. A comparative analysis of the momentum patterns on the substrates with different symmetry indicates that both constituents of the twofold degenerate FePc lowest unoccupied molecular orbital are occupied by charge transfer from the substrate at the interface. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2013.10.020
  • 2014 • 126 The adsorption behavior of octafluoropropane at the water/gas interface
    Giebel, F. and Paulus, M. and Nase, J. and Bieder, S. and Kiesel, I. and Tolan, M.
    Journal of Applied Physics 116 (2014)
    We studied the adsorption behavior of the gas octafluoropropane at the water/gas interface as a function of different pressures. In a custom-made measurement cell, the gas pressure was varied in a range between 1 bar and close to the condensation pressure of octafluoropropane. The electron density profiles of the adsorption layers show that the layer thickness increases with pressure. The evolution of the layer electron density indicates that the bulk electron density is reached if a layer consisting of more than one monolayer of octafluoropropane is adsorbed on the water surface. © 2014 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4902961
  • 2014 • 125 Low-temperature oxidation of carbon monoxide with gold(III) ions supported on titanium oxide
    Grünert, W. and Großmann, D. and Noei, H. and Pohl, M.-M. and Sinev, I. and De Toni, A. and Wang, Y. and Muhler, M.
    Angewandte Chemie - International Edition 53 3245-3249 (2014)
    Au/TiO2 catalysts prepared by a deposition-precipitation process and used for CO oxidation without previous calcination exhibited high, largely temperature-independent conversions at low temperatures, with apparent activation energies of about zero. Thermal treatments, such as He at 623 K, changed the conversion-temperature characteristics to the well-known S-shape, with activation energies slightly below 30 kJ mol-1. Sample characterization by XAFS and electron microscopy and a low-temperature IR study of CO adsorption and oxidation showed that CO can be oxidized by gas-phase O2 at 90 K already over the freeze-dried catalyst in the initial state that contained Au exclusively in the +3 oxidation state. CO conversion after activation in the feed at 303 K is due to AuIII-containing sites at low temperatures, while Au0 dominates conversion at higher temperatures. After thermal treatments, CO conversion in the whole investigated temperature range results from sites containing exclusively Au0. Ionic or metallic: Au3+ ions on TiO2 (see HAADF-STEM image of a freshly prepared sample) can catalyze the oxidation of CO at low temperatures. The reaction rates at Au3+-containing centers are similar to those found at metallic gold clusters. However, the apparent activation energies are very low, which is probably due to the opposing influence of the true activation energy and the adsorption enthalpy of CO on Au3+ centers. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201308206
  • 2014 • 124 Use of the capping agent for the electrochemical detection and quantification of nanoparticles: CdSe quantum dots
    Hepburn, W.G. and Batchelor-Mcauley, C. and Tschulik, K. and Kachoosangi, R.T. and Ness, D. and Compton, R.G.
    Sensors and Actuators, B: Chemical 204 445-449 (2014)
    The electrochemical detection of organic capped CdSe nanoparticles is achieved down to the highly dilute concentration of 15 pM. Herein, electrode modification is undertaken either via a simple and fast adsorption methodology, or by direct dropcasting of the material. Importantly, the adsorption of the CdSe nanoparticles is evidenced at higher surface coverages by the direct measurement of the cadmium reduction signal. A lower analytical detection limit for the CdSe nanoparticles is enabled by the enhancement of the diffusional borax reduction signal on a gold electrode modified with the quantum dots. The presence of a non-electroactive layer on an electrode has been shown to alter the apparent electrochemical rate constant via modifying the solubility and mass-transport of an electroactive species adjacent to the electrochemical interface. In the present case the origin of the enhanced rate of reduction for borax is ascribed as being due to the presence of the non-electroactive organic capping agent. Hence, due to the ubiquitous nature of capping agents within the field of nano-chemistry, the methodology represents a facile and generally applicable detection route. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.snb.2014.07.111
  • 2014 • 123 Using the first steps of hydration for the determination of molecular conformation of a single molecule
    Henzl, J. and Boom, K. and Morgenstern, K.
    Journal of the American Chemical Society 136 13341-13347 (2014)
    Determination of the exact structure of individual molecules is the ultimate goal of high-resolution microscopy. However, the resolution of scanning tunneling microscopy (STM) is intrinsically limited to the extent of molecular orbitals, which frequently do not differ for small changes in the molecular conformation. Here we use the position of water molecules during the first hydration steps of an azobenzene derivative on Au(111) to determine not only the orientation of the end groups with respect to the phenyl rings but also the orientation of the two phenyl rings with respect to the azo group. We investigate the co-adsorption of 4,4'-hydroxy-azobenzene and water molecules on Au(111) by low-temperature STM. The water molecules are attached exclusively to the hydroxyl end groups of the azobenzene derivatives. Predominantly the trans-azobenzene molecule with the two hydroxyl groups pointing into opposite directions is adsorbed. As corroborated by the attachment of a single water molecule to 4-anilino-4?-nitro azobenzene on the same inert surface, the method is generally applicable for structure determination of molecules with appropriate end groups. Our study thus gives unprecedented information about the intramolecular orientation based on the first real space observation of the hydration of a functional molecule. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja506762t
  • 2014 • 122 Temperature-driven adsorption and desorption of proteins at solid-liquid interfaces
    Kiesel, I. and Paulus, M. and Nase, J. and Tiemeyer, S. and Sternemann, C. and Rüster, K. and Wirkert, F.J. and Mende, K. and Büning, T. and Tolan, M.
    Langmuir 30 2077-2083 (2014)
    The heat-induced desorption and adsorption of the proteins lysozyme, ribonuclease A, bovine serum albumin, and fibronectin at protein layers was investigated in two different environments: pure buffer and protein solution. Using two different environments allows us to distinguish between thermodynamic and kinetic mechanisms in the adsorption process. We observed a desorption in buffer and an adsorption in protein solution, depending upon protein properties, such as size, stability, and charge. We conclude that the desorption in buffer is mainly influenced by the mobility of the proteins at the interface, while the adsorption in protein solution is driven by conformational changes and, thereby, a gain in entropy. These results are relevant for controlling biofilm formation at solid-liquid interfaces. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/la404884a
  • 2014 • 121 Periodic mesoporous organosilicas as adsorbents of toxic trace gases out of the ambient air
    Martens, S. and Ortmann, R. and Brieler, F.J. and Pasel, C. and Lee, Y.J. and Bathen, D. and Fröba, M.
    Zeitschrift fur Anorganische und Allgemeine Chemie 640 632-640 (2014)
    New periodic mesoporous organosilicas were synthesized using [bis(triethoxysilyl)vinyl]aniline (BTEVA) and [bis(triethoxysilyl)vinyl]benzene (BTEVB) as precursors and by functionalizing the BTEVA PMO with aminopropyl groups. All PMO materials showed a high degree of hexagonal order in the respective P-XRDs as well as high surface areas with narrow pore size distributions. The PMOs together with activated carbon were investigated with regards to their hexanal adsorption capacity in a gravimetric apparatus with an attached GC-MS. For the amine PMOs two different regions during adsorption were found. In the first region loadings in the range of 30 wt% were reached by a fast adsorption process. In the second region the loading reached 50 wt% but with slower kinetics. In desorption experiments up to 150 °C the loading decreased from 50 wt% to 30 wt%, where an equilibrium state was observed. With GC-MS and NMR the first region could be assigned to a chemisorption process, where the hexanal forms an imine group with the amine function in a Schiff base reaction. The subsequent second region can be assigned to weaker and thus reversible physisorption. The total capacity (first and second region) is substantially higher for the amine PMOs than for the activated carbon. Due to the chemical bonding in the chemisorption process, the desorption behavior is completely different: compared to the activated carbon the bonding of hexanal is much stronger for the amine PMOs. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/zaac.201300519
  • 2014 • 120 Adsorption of Colloidal Platinum Nanoparticles to Supports: Charge Transfer and Effects of Electrostatic and Steric Interactions
    Marzun, G. and Streich, C. and Jendrzej, S. and Barcikowski, S. and Wagener, P.
    Langmuir 30 11928-11936 (2014)
    Adsorption of colloidal nanoparticles to surfaces and supports is a convenient approach to heterogeneous catalysts, polymer additives, or wastewater treatment. We investigated the adsorption efficiency of laser-generated and initially ligand-free platinum nanoparticles to TiO<inf>2</inf> supports as a function of pH, ionic strength, and ligand surface coverage. The nanoparticle adsorption is dominantly controlled by electrostatic interactions: if the pH of the suspension is between the isoelectric point of the nanoparticles and the support, nanoparticles are adsorbed and transfer a net charge to the support. This charge-driven adsorption is not affected by steric repulsion due to various ligands attached to the nanoparticle surface. In addition to electrostatic interactions, colloidal stability given by moderate ionic strengths and pH values above the isoelectric point of nanoparticles are prerequisites for colloidal deposition. (Chemical Equation Presented). © 2014 American Chemical Society.
    view abstractdoi: 10.1021/la502588g
  • 2014 • 119 Molecular-Scale Imaging of Water Near Charged Surfaces
    Mehlhorn, M. and Schnur, S. and Groß, A. and Morgenstern, K.
    ChemElectroChem 1 431-435 (2014)
    The orientation of water molecules on water bilayers is investigated on Cu(111) by a combination of scanning tunneling microscopy and density functional theory. Theory predicts that the application of a field reorients the adsorbed water molecules at a distance of close to a nanometer from the surface. Experimental evidence is presented for this prediction. Furthermore, the process differs strongly between adsorption on two and on three ordered layers. We propose that these results give insight into the behavior of the diffusive layer close to electrodes. So simple? Since the basic idea of ultrahigh-vacuum (UHV) electrochemical modeling emerged, it has been claimed that UHV model experiments are too simple because they do not include the electrode potential. This combined scanning tunneling microscopy and density functional theory study gives insight into the influence of the electric field on single molecules in the diffusive layer. A field reorients adsorbed water molecules on water bilayers on Cu(111) at a distance of about 1nm from the surface. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201300063
  • 2014 • 118 In situ non-DLVO stabilization of surfactant-free, plasmonic gold nanoparticles: Effect of Hofmeister's anions
    Merk, V. and Rehbock, C. and Becker, F. and Hagemann, U. and Nienhaus, H. and Barcikowski, S.
    Langmuir 30 4213-4222 (2014)
    Specific ion effects ranking in the Hofmeister sequence are ubiquitous in biochemical, industrial, and atmospheric processes. In this experimental study specific ion effects inexplicable by the classical DLVO theory have been investigated at curved water-metal interfaces of gold nanoparticles synthesized by a laser ablation process in liquid in the absence of any organic stabilizers. Notably, ion-specific differences in colloidal stability occurred in the Hückel regime at extraordinarily low salinities below 50 μM, and indications of a direct influence of ion-specific effects on the nanoparticle formation process are found. UV-vis, zeta potential, and XPS measurements help to elucidate coagulation properties, electrokinetic potential, and the oxidation state of pristine gold nanoparticles. The results clearly demonstrate that stabilization of ligand-free gold nanoparticles scales proportionally with polarizability and antiproportionally with hydration of anions located at defined positions in a direct Hofmeister sequence of anions. These specific ion effects might be due to the adsorption of chaotropic anions (Br-, SCN-, or I-) at the gold/water interface, leading to repulsive interactions between the partially oxidized gold particles during the nanoparticle formation process. On the other hand, kosmotropic anions (F - or SO4 2-) seem to destabilize the gold colloid, whereas Cl- and NO3 - give rise to an intermediate stability. Quantification of surface charge density indicated that particle stabilization is dominated by ion adsorption and not by surface oxidation. Fundamental insights into specific ion effects on ligand-free aqueous gold nanoparticles beyond purely electrostatic interactions are of paramount importance in biomedical or catalytic applications, since colloidal stability appears to depend greatly on the type of salt rather than on the amount. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/la404556a
  • 2014 • 117 A critical study of the use of the iodine number in the characterization of activated carbons
    Möller, M. and Treese, J. and Pasel, C. and Bathen, D.
    Chemie-Ingenieur-Technik 86 67-71 (2014)
    In industrial practice the iodine number is often used to characterize the adsorptive properties of activated carbons in liquid phase applications. Manufacturers of activated carbons determine the iodine number according to different standards (e.g., CEFIC 2.3 and ASTMD4607). This leads to different experimental protocols being applied to measure the same number. Therefore, activated carbons made by different manufacturers are difficult to compare. This work presents a systematic comparison and a critical assessment of experimental protocols used to determine the iodine number. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201300108
  • 2014 • 116 Dynamic surface processes of nanostructured Pd2Ga catalysts derived from hydrotalcite-like precursors
    Ota, A. and Kröhnert, J. and Weinberg, G. and Kasatkin, I. and Kunkes, E.L. and Ferri, D. and Girgsdies, F. and Hamilton, N. and Armbrüster, M. and Schlögl, R. and Behrens, M.
    ACS Catalysis 4 2048-2059 (2014)
    The stability of the surface termination of intermetallic Pd2Ga nanoparticles and its effect on the hydrogenation of acetylene was investigated. For this purpose, a precursor synthesis approach was applied to synthesize supported intermetallic Pd2Ga nanoparticles. A series of Pd-substituted MgGa-hydrotalcite (HT)-like compounds with different Pd loading was prepared by coprecipitation and studied in terms of loading, phase formation, stability and catalytic performance in the selective hydrogenation of acetylene. Higher Pd loadings than 1 mol % revealed an incomplete incorporation of Pd into the HT lattice, as evidenced by XANES and TPR measurements. Upon thermal reduction in hydrogen, Pd2Ga nanoparticles were obtained with particle sizes varying with the Pd loading, from 2 nm to 6 nm. The formation of intermetallic Pd2Ga nanoparticles led to a change of the CO adsorption properties as was evidenced by IR spectroscopy. Dynamic changes of the surface were noticed at longer exposure times to CO and higher coverage at room temperature as a first indication of surface instability. These were ascribed to the decomposition into a Ga-depleted Pd phase and Ga 2O3, which is a process that was suppressed at liquid nitrogen temperature. The reduction of the Pd precursor at 473 K is not sufficient to form the Pd2Ga phase and yielded a poorly selective catalyst (26% selectivity to ethylene) in the semihydrogenation of acetylene. In accordance with the well-known selectivity-promoting effect of a second metal, the selectivity was increased to 80% after reduction at 773 K due to a change from the elemental to the intermetallic state of palladium in our catalysts. Interestingly, if air contact was avoided after reduction, the conversion slowly rose from initially 22% to 94% with time on stream. This effect is interpreted in the light of chemical response of Pd and Pd2Ga to the chemical potential of the reactive atmosphere. Conversely to previous interpretations, we attribute the initial low active state to the clean intermetallic surface, while the increase in conversion is related to the surface decomposition of the Pd2Ga particles. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/cs500465r
  • 2014 • 115 Polymorphic calcium carbonate phases as adsorbents for allergens in natural rubber latex
    Petrack, J. and Vucak, M. and Nover, C. and Epple, M.
    Journal of Applied Polymer Science 132 (2014)
    Natural rubber latex contains different allergenic proteins and peptides that restrict its application in consumer products. Small mineral particles have a comparatively high specific surface area and are therefore well suited to adsorb such biomolecules. The adsorption of such biomolecules onto different polymorphic phases of calcium carbonate, i.e., calcite, aragonite, and vaterite, was quantitatively determined, both from solution as well as from cured natural rubber latex. All phases were able to adsorb the allergens, with slight differences between the different allergens. Desorption experiments showed differences between the allergens of natural rubber latex, but only small differences between the polymorphic phases of calcium carbonate. The release of the allergens from latex objects with incorporated calcium carbonate particles showed that a retention of allergens is possible by adding calcium carbonate as a filler material. © 2014 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/app.41271
  • 2014 • 114 Targeted manipulation of metal-organic frameworks to direct sorption properties
    Schneemann, A. and Henke, S. and Schwedler, I. and Fischer, R.A.
    ChemPhysChem 15 823-839 (2014)
    Metal-organic frameworks are promising materials for manifold applications. This Minireview highlights approaches for the fine-tuning of specific sorption properties (e.g. capacity, selectivity, and breathing behavior) of this interesting class of materials. Central aspects covered are the control over the crystal morphology, the targeted tuning of sorption properties by judicious choice of metal centers and linkers, and the preparation of host-guest systems. We want to introduce the reader to these topics on the basis of the manipulation of a handful of outstanding prototypical metal-organic frameworks. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201300976
  • 2014 • 113 Photothermal laser fabrication of micro- and nanostructured chemical templates for directed protein immobilization
    Schröter, A. and Franzka, S. and Hartmann, N.
    Langmuir 30 14841-14848 (2014)
    Photothermal patterning of poly(ethylene glycol) terminated organic monolayers on surface-oxidized silicon substrates is carried out using a microfocused beam of a CW laser operated at a wavelength of 532 nm. Trichlorosilane and trimethoxysilane precursors are used for coating. Monolayers from trimethoxysilane precursors show negligible unspecific protein adsorption in the background, i.e., provide platforms of superior protein repellency. Laser patterning results in decomposition of the monolayers and yields chemical templates for directed immobilization of proteins at predefined positions. Characterization is carried out via complementary analytical methods including fluorescence microscopy, atomic force microscopy, and scanning electron microscopy. Appropriate labeling techniques (fluorescent markers and gold clusters) and substrates (native and thermally oxidized silicon substrates) are chosen in order to facilitate identification of protein adsorption and ensure high sensitivity and selectivity. Variation of the laser parameters at a 1/e2 spot diameter of 2.8 μm allows for fabrication of protein binding domains with diameters on the micrometer and nanometer length scale. Minimum domain sizes are about 300 nm. In addition to unspecific protein adsorption on as-patterned monolayers, biotin-streptavidin coupling chemistry is exploited for specific protein binding. This approach represents a novel facile laser-based means for fabrication of protein micro- and nanopatterns. The routine is readily applicable to femtosecond laser processing of glass substrates for the fabrication of transparent templates. (Graph Presented). © 2014 American Chemical Society.
    view abstractdoi: 10.1021/la503814n
  • 2014 • 112 Retarding the corrosion of iron by inhomogeneous magnetic fields
    Sueptitz, R. and Tschulik, K. and Uhlemann, M. and Eckert, J. and Gebert, A.
    Materials and Corrosion 65 803-808 (2014)
    The influence of a magnetic field yielding high magnetic flux densities and high flux density gradients on the free corrosion behavior of iron in a low concentrated acidic solution with and without chloride ions is studied by long time exposure experiments and electrochemical impedance spectroscopy (EIS). A decrease of the corrosion rate in electrode surface regions of high magnetic flux density is detected. This decrease of the dissolution rate is significantly stronger in the presence of chloride ions. The observed effects are discussed on the basis of the magnetically induced forces acting on the ions present in the solution, the surface coverage fraction of adsorbed species, and the stability of these adsorbed species. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/maco.201206890
  • 2014 • 111 Gold nanoparticles interfere with sperm functionality by membrane adsorption without penetration
    Taylor, U. and Barchanski, A. and Petersen, S. and Kues, W.A. and Baulain, U. and Gamrad, L. and Sajti, L. and Barcikowski, S. and Rath, D.
    Nanotoxicology 8 118-127 (2014)
    To examine gold nanoparticle reprotoxicity, bovine spermatozoa were challenged with ligand-free or oligonucleotide-conjugated gold nanoparticles synthesized purely without any surfactants by laser ablation. Sperm motility declined at nanoparticle mass dose of 10 μg/ml (corresponding to ∼14 000 nanoparticles per sperm cell) regardless of surface modification. Sperm morphology and viability remained unimpaired at all concentrations. Transmission electron microscopy showed an modification dependant attachment of nanoparticles to the cell membrane of spermatozoa, but provided no evidence for nanoparticle entrance into sperm cells. A molecular examination revealed a reduction of free thiol residues on the cell membrane after nanoparticle exposure, which could explain the decrease in sperm motility. Sperm fertilising ability decreased after exposure to 10 μg/ml of ligand-free nanoparticles indicating that agglomerated ligand-free nanoparticles interfere with membrane properties necessary for fertilisation. In conclusion, nanoparticles may impair key sperm functions solely by interacting with the sperm surface membrane. © 2014 Informa UK Ltd. All rights reserved.
    view abstractdoi: 10.3109/17435390.2013.859321
  • 2014 • 110 Conditioning of self-assembled monolayers at two static immersion test sites along the east coast of Florida and its effect on early fouling development
    Thome, I. and Bauer, S. and Vater, S. and Zargiel, K. and Finlay, J.A. and Arpa-Sancet, M.P. and Alles, M. and Callow, J.A. and Callow, M.E. and Swain, G.W. and Grunze, M. and Rosenhahn, A.
    Biofouling 30 1011-1021 (2014)
    Among the first events after immersion of surfaces in the ocean is surface ‘conditioning’. Here, the accumulation and composition of the conditioning films formed after immersion in the ocean are analyzed. In order to account for different surface chemistries, five self-assembled monolayers that differ in resistance to microfouling and wettability were used. Water samples from two static immersion test sites along the east coast of Florida were collected at two different times of the year and used for experiments. Spectral ellipsometry revealed that conditioning films were formed within the first 24 h and contact angle goniometry showed that these films changed the wettability and rendered hydrophobic surfaces more hydrophilic and vice versa. Infrared reflection adsorption spectroscopy showed that the composition of the conditioning film depended on both the wettability and immersion site. Laboratory and field assays showed that the presence of a conditioning film did not markedly influence settlement of microorganisms. © 2014, © 2014 Taylor & Francis.
    view abstractdoi: 10.1080/08927014.2014.957195
  • 2014 • 109 Position of Cu atoms at the Pt(111) electrode surfaces controls electrosorption of (H)SO4 (2)- from H2SO4 electrolytes
    Tymoczko, J. and Schuhmann, W. and Bandarenka, A.S.
    ChemElectroChem 1 (2014)
    Selective positioning of monolayer amounts of foreign atoms at the surface and subsurface regions of metal electrodes is a promising way to fine-tune the properties of the electrode/ electrolyte interface. The latter is critical as it largely governs the adsorption of electrolyte components and reaction intermediates and, therefore, controls many key electrocatalytic processes. Using model Pt(111) single-crystal electrodes, we demonstrate how the relative position of Cu atoms at the surface drastically changes the adsorption energies for (bi)sulfate anions. Our measurements involve pseudomorphic overlayers of Cu on Pt(111) as well as Cu-Pt(111) surface and sub-surface alloys, where Cu atoms were located either in the first or in the second atomic layers of Pt, respectively. In the case of Cu- Pt(111) surface alloys, specific adsorption of the anions starts earlier compared to the unmodified Pt(111) surface. In contrast, placing Cu atoms into the second atomic layer weakens the binding between the surface and the anions. Surprisingly, Cu pseudomorphic overlayers do not reveal any specific adsorption of (bi)sulfates (within the region of the overlayer stability). Taking into account that electrified interfaces between Pt(111) electrodes and sulfate-containing electrolytes often play the role of benchmark systems in fundamental physico-chemical and, particularly, electrocatalytic studies, our findings demonstrate a promising and relatively easy route of tuning the properties of these interfaces. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201300107
  • 2014 • 108 X-ray reflectivity measurements of liquid/solid interfaces under high hydrostatic pressure conditions
    Wirkert, F.J. and Paulus, M. and Nase, J. and Möller, J. and Kujawski, S. and Sternemann, C. and Tolan, M.
    Journal of Synchrotron Radiation 21 76-81 (2014)
    A high-pressure cell for in situ X-ray reflectivity measurements of liquid/solid interfaces at hydrostatic pressures up to 500 MPa (5 kbar), a pressure regime that is particularly important for the study of protein unfolding, is presented. The original set-up of this hydrostatic high-pressure cell is discussed and its unique properties are demonstrated by the investigation of pressure-induced adsorption of the protein lysozyme onto hydrophobic silicon wafers. The presented results emphasize the enormous potential of X-ray reflectivity studies under high hydrostatic pressure conditions for the in situ investigation of adsorption phenomena in biological systems.© 2014 International Union of Crystallography.
    view abstractdoi: 10.1107/S1600577513021516
  • 2014 • 107 Stable zinc oxide nanoparticle dispersions in ionic liquids
    Wittmar, A. and Gautam, D. and Schilling, C. and Dörfler, U. and Mayer-Zaika, W. and Winterer, M. and Ulbricht, M.
    Journal of Nanoparticle Research 16 (2014)
    The influence of the hydrophilicity and length of the cation alkyl chain in imidazolium-based ionic liquids on the dispersability of ZnO nanoparticles by ultrasound treatment was studied by dynamic light scattering and advanced rheology. ZnO nano-powder synthesized by chemical vapor synthesis was used in parallel with one commercially available material. Before preparation of the dispersion, the nanoparticles characteristics were determined by transmission electron microscopy, X-ray diffraction, nitrogen adsorption with BET analysis, and FT-IR spectroscopy. Hydrophilic ionic liquids dispersed all studied nanopowders better and in the series of hydrophilic ionic liquids, an improvement of the dispersion quality with increasing length of the alkyl chain of the cation was observed. Especially, for ionic liquids with short alkyl chain, additional factors like nanoparticle concentration in the dispersion and the period of the ultrasonic treatment had significant influence on the dispersion quality. Additionally, nanopowder characteristics (crystallite shape and size as well as the agglomeration level) influenced the dispersion quality. The results indicate that the studied ionic liquids are promising candidates for absorber media at the end of the gas phase synthesis reactor allowing the direct preparation of non-agglomerated nanoparticle dispersions without supplementary addition of dispersants and stabilizers. © Springer Science+Business Media 2014.
    view abstractdoi: 10.1007/s11051-014-2341-2
  • 2013 • 106 Internal detection of surface plasmon coupled chemiluminescence during chlorination of potassium thin films
    Becker, F. and Krix, D. and Hagemann, U. and Nienhaus, H.
    Journal of Chemical Physics 138 (2013)
    The interaction of chlorine with potassium surfaces is a prototype reaction with a strong non-adiabatic energy transfer leading to exoemission and chemiluminescence. Thin film K/Ag/p-Si(111) Schottky diodes with 8 nm potassium on a 5-200 nm thick Ag layer are used as 2π-photodetectors for the chemiluminescence during chlorination of the K film at 110 K. The observed photocurrent shows a sharp maximum for small exposures and decreases gradually with the increasing chloride layer. The time dependence can be explained by the reaction kinetics, which is governed initially by second-order adsorption processes followed by an electric field-assisted diffusion. The detector current corresponds to a yield of a few percent of elementary charge per reacting chlorine molecule and is orders of magnitude larger than for external detection. The photoyield can be enhanced by increasing the Ag film thickness. For Ag films of 30 and 50 nm, the yield exhibits a maximum indicating surface plasmon coupled chemiluminescence. Surface plasmon polaritons in the Ag layer are excited by the reaction and decay radiatively into Si leading to the observed currents. A model calculation for the reverse process in attenuated total reflection is applied to explain the observed current yield maxima. © 2013 American Institute of Physics.
    view abstractdoi: 10.1063/1.4776156
  • 2013 • 105 Adsorption of amino acids on the magnetite-(111)-surface: A force field study
    Bürger, A. and Magdans, U. and Gies, H.
    Journal of Molecular Modeling 19 851-857 (2013)
    Magnetite (Fe3O4) is an important biomineral, e.g., used by magnetotactic bacteria. The connection between the inorganic magnetite-(111)-surface and the organic parts of the bacteria is the magnetosome membrane. The membrane is built by different magnetosome membrane proteins (MMPs), which are dominated by the four amino acids glycine (Gly), aspartic acid (Asp), leucine (Leu) and glutamic acid (Glu). Force field simulations of the interaction of the magnetite-(111)-surface and the main amino acid compounds offer the possibility to investigate if and how the membrane proteins could interact with the mineral surface thus providing an atomistic view on the respective binding sites. In a force field simulation the four amino acids were docked on the Fe-terminated magnetite-(111)-surface. The results show that it is energetically favorable for the amino acids to adsorb on the surface with Fe-O-distances between 2.6 Å and 4.1 Å. The involved O-atoms belong to the carboxyl-group (Asp and Glu) or to the carboxylate-group (Gly, Leu and Glu). Electrostatic interactions dominate the physisorption of the amino acids. During the simulations, according to the frequency of the best results, the global minimum for the docking interaction could be attained for all amino acids analyzed. © 2012 Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00894-012-1606-x
  • 2013 • 104 Upscaling the chemical vapor infiltration process of activated carbon with TMS
    Curdts, B. and Helmich, M. and Pasel, C. and Bathen, D. and Atakan, B. and Pflitsch, C.
    Physics Procedia 46 248-254 (2013)
    Activated carbons are important adsorbents covering a broad range of applications from gas to air purification. In order to improve their mechanical stability and their resistance against oxidation they are infiltrated here with an inert material, SiC or SiO2. For this process the chemical vapor infiltration technique with tetramethylsilane as precursor is used. The process is designed for the infiltration of larger quantities (up to 50 g) which allows further analysis of the materials produced: in comparison to the uncoated activated carbon the novel adsorbent have an increased breaking strength and the BET-surface areas and pore volumes are on high levels. The possibility of using the novel material for solvent adsorption is demonstrated for acetone. © 2013 The Authors.
    view abstractdoi: 10.1016/j.phpro.2013.07.061
  • 2013 • 103 Blocked-micropores, surface functionalized, bio-compatible and silica-coated iron oxide nanocomposites as advanced MRI contrast agent
    Darbandi, M. and Laurent, S. and Busch, M. and Li, Z.-A. and Yuan, Y. and Krüger, M. and Farle, M. and Winterer, M. and Vander Elst, L. and Muller, R.N. and Wende, H.
    Journal of Nanoparticle Research 15 (2013)
    Biocompatible magnetic nanoparticles have been found promising in several biomedical applications for tagging, imaging, sensing and separation in recent years. In this article, a systematic study of the design and development of surface-modification schemes for silica-coated iron oxide nanoparticles (IONP) via a one-pot, in situ method at room temperature is presented. Silica-coated IONP were prepared in a water-in-oil microemulsion, and subsequently the surface was modified via addition of organosilane reagents to the microemulsion system. The structure and the morphology of the as synthesized nanoparticles have been investigated by means of transmission electron microscopy (TEM) and measurement of N2 adsorption-desorption. Electron diffraction and high-resolution transmission electron microscopic (TEM) images of the nanoparticles showed the highly crystalline nature of the IONP structures. Nitrogen adsorption indicates microporous and blocked-microporous structures for the silica-coated and amine functionalized silica-coated IONP, respectively which could prove less cytotoxicity of the functionalized final product. Besides, the colloidal stability of the final product and the presence of the modified functional groups on top of surface layer have been proven by zeta-potential measurements. Owing to the benefit from the inner IONP core and the hydrophilic silica shell, the as-synthesized nanocomposites were exploited as an MRI contrast enhancement agent. Relaxometric results prove that the surface functionalized IONP have also signal enhancement properties. These surface functionalized nanocomposites are not only potential candidates for highly efficient contrast agents for MRI, but could also be used as ultrasensitive biological-magnetic labels, because they are in nanoscale size, having magnetic properties, blocked-microporous and are well dispersible in biological environment. © 2013 Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s11051-013-1664-8
  • 2013 • 102 Accessing 4f-states in single-molecule spintronics
    Fahrendorf, S. and Atodiresei, N. and Besson, C. and Caciuc, V. and Matthes, F. and Blügel, S. and Kögerler, P. and Bürgler, D.E. and Schneider, C.M.
    Nature Communications 4 (2013)
    Magnetic molecules are potential functional units for molecular and supramolecular spintronic devices. However, their magnetic and electronic properties depend critically on their interaction with metallic electrodes. Charge transfer and hybridization modify the electronic structure and thereby influence or even quench the molecular magnetic moment. Yet, detection and manipulation of the molecular spin state by means of charge transport, that is, spintronic functionality, mandates a certain level of hybridization of the magnetic orbitals with electrode states. Here we show how a judicious choice of the molecular spin centres determines these critical molecule-electrode contact characteristics. In contrast to late lanthanide analogues, the 4f-orbitals of single bis(phthalocyaninato)-neodymium(III) molecules adsorbed on Cu(100) can be directly accessed by scanning tunnelling microscopy. Hence, they contribute to charge transport, whereas their magnetic moment is sustained as evident from comparing spectroscopic data with ab initio calculations. Our results showcase how tailoring molecular orbitals can yield all-electrically controlled spintronic device concepts. © 2013 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms3425
  • 2013 • 101 Long-circulating poly(ethylene glycol)-coated poly(lactid-co-glycolid) microcapsules as potential carriers for intravenously administered drugs
    Ferenz, K.B. and Waack, I.N. and Mayer, C. and De Groot, H. and Kirsch, M.
    Journal of Microencapsulation 30 632-642 (2013)
    The intrinsic advantages of microcapsules with regard to nanocapsules as intravenous drug carrier systems are still not fully exploited. Especially, in clinical situations where a long-term drug release within the vascular system is desired, if large amounts of drug have to be administered or if capillary leakage occurs, long-circulating microparticles may display a superior alternative to nanoparticles. Here, microcapsules were synthesised and parameters such as in vitro tendency of agglomeration, protein adsorption and in vivo performance were investigated. Biocompatible poly(ethylene glycol) (PEG)-coated poly(DL-lactide-co-glycolide) (PLGA) as wall material, solid and perfluorodecalin (PFD)-filled PEG-PLGA microcapsules (1.5 μm diameter) were manufactured by using a modified solvent evaporation method with either 1% poly(vinyl alcohol) (PVA) or 1.5% cholate as emulsifying agents. Compared to microcapsules manufactured with cholate, the protein adsorption (albumin and IgG) was clearly decreased and agglomeration of capsules was prevented, when PVA was used. The intravenous administration of these microcapsules, both solid and PFD-filled, in rats was successful and exhibited a circulatory half-life of about 1 h. Our data clearly demonstrate that PEG-PLGA microcapsules, manufactured by using PVA, are suitable biocompatible, long-circulating drug carriers, applicable for intravenous administration. © 2013 Informa UK Ltd. All rights reserved.
    view abstractdoi: 10.3109/02652048.2013.770098
  • 2013 • 100 Enhancing the activity of Pd on carbon nanofibers for deoxygenation of amphiphilic fatty acid molecules through support polarity
    Gosselink, R.W. and Xia, W. and Muhler, M. and De Jong, K.P. and Bitter, J.H.
    ACS Catalysis 3 2397-2402 (2013)
    The influence of support polarity on Pd/CNF for the deoxygenation of fatty acids was studied. Catalysts with a low (O/C = 3.5 × 10-2 at/at from X-ray photoelectron spectroscopy (XPS)) and a high (O/C = 5.9 × 10-2 at/at from XPS) amount of oxygen containing groups on the support were prepared. The latter were introduced via a HNO3 gas phase oxidation treatment on Pd loaded supports. The presence of oxygen containing groups was beneficial for the activity of Pd for the deoxygenation of the amphiphilic stearic acid. This is attributed to a favorable mode of adsorption of the reactant via the carboxylic acid group on the more polar support in the vicinity of the catalytically active Pd nanoparticles. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/cs400478q
  • 2013 • 99 Reorientation of a single bond within an adsorbed molecule by tunneling electrons
    Henzl, J. and Boom, K. and Morgenstern, K.
    Journal of the American Chemical Society 135 11501-11504 (2013)
    Scanning tunneling microscopy offers the exciting possibility to manipulate individual molecules by vibrational excitation via inelastically tunneling electrons. The electrons transfer energy into molecular vibrational modes, leading to breakage or formation of individual bonds. It is challenging to precisely control intramolecular changes by this process. We demonstrate that for 4,4′-dihydroxyazobenzene adsorbed on Au(111) or Ag(111), the manipulation facilitates rotation of the OH end groups around the C-O bond between metastable states; this corresponds to a reorientation of the hydrogen, the ultimate limit of a conformational change within a molecule. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ja405809f
  • 2013 • 98 Controlling adsorption of semiflexible polymers on planar and curved substrates
    Kampmann, T.A. and Boltz, H.-H. and Kierfeld, J.
    Journal of Chemical Physics 139 (2013)
    We study the adsorption of semiflexible polymers such as polyelectrolytes or DNA on planar and curved substrates, e.g., spheres or washboard substrates via short-range potentials using extensive Monte Carlo simulations, scaling arguments, and analytical transfer matrix techniques. We show that the adsorption threshold of stiff or semiflexible polymers on a planar substrate can be controlled by polymer stiffness: adsorption requires the highest potential strength if the persistence length of the polymer matches the range of the adsorption potential. On curved substrates, i.e., an adsorbing sphere or an adsorbing washboard surface, the adsorption can be additionally controlled by the curvature of the surface structure. The additional bending energy in the adsorbed state leads to an increase of the critical adsorption strength, which depends on the curvature radii of the substrate structure. For an adsorbing sphere, this gives rise to an optimal polymer stiffness for adsorption, i.e., a local minimum in the critical potential strength for adsorption, which can be controlled by curvature. For two- and three-dimensional washboard substrates, we identify the range of persistence lengths and the mechanisms for an effective control of the adsorption threshold by the substrate curvature. © 2013 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4813021
  • 2013 • 97 Al-induced faceting of Si(113)
    Klein, C. and Heidmann, I. and Nabbefeld, T. and Speckmann, M. and Schmidt, T. and Meyer zu Heringdorf, F.-J. and Falta, J. and Horn-Von Hoegen, M.
    Surface Science 618 109-114 (2013)
    Adsorption of Al on a Si(113) substrate at elevated temperatures causes a faceting transition of the initially flat surface. The (113) surface decomposes into a quasi-periodic sequence of Al terminated (115)- and (112)-facets. The resulting surface morphology is characterized in-situ by reciprocal space maps obtained with in-situ spot profile analyzing low-energy electron diffraction and ex-situ atomic force microscopy. The periodicity length of the faceted surface increases with adsorption temperature from 7 nm at 650 C to 80 nm at 800 C. The stability of the Al terminated Si(112) surface is the driving force for the faceting transition. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.susc.2013.08.007
  • 2013 • 96 Novel antifouling positively charged hybrid ultrafiltration membranes for protein separation based on blends of carboxylated carbon nanotubes and aminated poly(arylene ether sulfone)
    Kumar, M. and Ulbricht, M.
    Journal of Membrane Science 448 62-73 (2013)
    In this study, an aminated hydrophilic poly(arylene ether sulfone) (APAES) multiblock copolymer was prepared from previously synthesized bromomethylated poly(arylene ether sulfone) (PAES-CH2Br) block copolymer via in situ amination with triethanolamine. Novel positively charged hybrid ultrafiltration membranes were fabricated by film casting with non-solvent induced phase separation of blends of PAES and APAES block copolymers with carboxylated multiwalled carbon nanotubes (MWCNT-COOH). Fourier transform infrared spectroscopy in the attenuated total reflection mode, scanning electron microscopy as well as contact angle and outer surface zeta potential studies were performed to characterize the membranes in detail. The results confirmed that the fabricated membranes were porous, hydrophilic, positively charged, and had improved antifouling capacity. The hybrid membranes were used in ultrafiltration of ovalbumin and lysozyme model solutions (individually) at varied pH values. Membrane performance depended on the contents of MWCNT-COOH, which could be adjusted by varying its fraction in the membrane casting solutions. Due to the combined effects of size exclusion and charge repulsion, the permeability, antifouling properties and separation selectivity of the hybrid ultrafiltration membranes could be improved simultaneously by increasing of charge density and fraction of MWCNT-COOH. © 2013.
    view abstractdoi: 10.1016/j.memsci.2013.07.055
  • 2013 • 95 Effect of adsorbed magnetic and non-magnetic atoms on electronic transport through surfaces with strong spin-orbit coupling
    Lükermann, D. and Sologub, S. and Pfnür, H. and Klein, C. and Horn-Von Hoegen, M. and Tegenkamp, C.
    Materialwissenschaft und Werkstofftechnik 44 210-217 (2013)
    Adsorption-induced reduction of the surface state conductivity in epitaxial Bi(111) films, a prototype system with a large Rashba-induced surface state splitting by adsorbed atoms of Bi, Fe and Co was investigated by macroscopic surface magneto-transport measurements at a temperature of 10 K. A detailed analysis of magneto-transport, DC-transport and Hall data reveals that the scattering efficiencies for Co and Fe are by a factor of two larger than for Bi. While for the latter, charge transfer and change of band filling near the Fermi level is negligible, we found an increase of hole concentration upon Co and Fe adsorption. These atoms act as acceptors and retract roughly 0.5 electrons from the surface per adsorbed atom. Besides the dominant classical magneto-conductance signal the films show signatures of weak anti-localization (WAL) reflecting the strong spin-orbit coupling in Bi(111) surface states. Our measurements show that the control of hybridization is important in order to make use of local spin-moments and to increase the backscattering rate in strongly spin-orbit coupled systems, e. g., topological insulators. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/mawe.201300114
  • 2013 • 94 DFT+ U study of arsenate adsorption on FeOOH surfaces: Evidence for competing binding mechanisms
    Otte, K. and Schmahl, W.W. and Pentcheva, R.
    Journal of Physical Chemistry C 117 15571-15582 (2013)
    On the basis of periodic density functional theory (DFT) calculations including an on-site Coulomb repulsion term U, we study the adsorption mechanism of arsenate on the goethite (101), akaganeite (100), and lepidocrocite (010) surfaces. Mono- and bidentate binding configurations of arsenate complexes are considered at two distinct iron surface sites - directly at 5-fold coordinated Fe1 and/or 4-fold coordinated Fe2 as well as involving ligand exchange. The results obtained within ab initio thermodynamics shed light on the ongoing controversy on the arsenate adsorption configuration, and we identify monodentate adsorbed arsenate complexes as stable configurations at ambient conditions with a strong preference for protonated arsenate complexes: a monodentate mononuclear complex at Fe1 (dFe1-As = 3.45 Å) at goethite (101) and a monodentate binuclear complex at Fe2 (dFe2-As = 3.29 Å) at akaganeite (100). Repulsive interactions between the complexes limit the loading capacity and promote configurations with maximized distances between the adsorbates. With decreasing oxygen pressures, a mixed adsorption of bidentate binuclear complexes at Fe1 (dFe1-As = 3.26-3.34 Å) and monodentate binuclear arsenate at Fe2 (dFe2-As = 3.31-3.50 Å) and, finally, rows of protonated bidentate complexes at Fe1 with d Fe1-As = 3.55-3.59 Å are favored at α-FeOOH(101) and β-FeOOH(100). At lepidocrocite (010) with only Fe2 sites exposed, the surface phase diagram is dominated by alternating protonated monodentate binuclear complexes (dFe2-As = 3.38 Å) and hydroxyl groups. At low oxygen pressures, alternating rows of protonated bidentate mononuclear complexes (dFe2-As = 3.10 Å) and water are present. Hydrogen bond formation to surface hydroxyl groups and water plays a crucial role in the stabilization of these adsorbate configurations and together with steric effects of the oxygen lone pairs leads to tilting of the arsenate complex that significantly reduces the Fe-As distance. Our results show that the Fe-As bond length is mainly determined by the protonation state, arsenate coverage, steric effects, and hydrogen bonding to surface functional groups and to a lesser extent by the adsorption mode. This demonstrates that the Fe-As distance cannot be used as a unique criterion to discriminate between adsorption modes. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/jp400649m
  • 2013 • 93 Lignocellulosic jute fiber as a bioadsorbent for the removal of azo dye from its aqueous solution: Batch and column studies
    Roy, A. and Chakraborty, S. and Kundu, S.P. and Adhikari, B. and Majumder, S.B.
    Journal of Applied Polymer Science 129 15-27 (2013)
    The feasibility of the use of jute fiber for the adsorption of azo dye from an aqueous solution was evaluated with batch and fixed-bed column studies. The batch studies illustrated that dye uptake was highly dependent on different process variables, namely, the pH, initial dye concentration of the solution, adsorbent dosage, contact time, ionic strength, and temperature. The exothermic and spontaneous nature of adsorption was revealed from thermodynamic study. The equilibrium adsorption data were highly consistent with the Langmuir isotherm and yielded an R2 value of 0.999. Kinetic studies divulged that the adsorption followed a pseudo-second-order model with regard to the intraparticle diffusion. In the column studies, the total amount of adsorbed dye and the adsorption capacity decreased with increasing flow rate and increased with increasing bed height and initial dye concentration. Also, the breakthrough time and exhaustion time increased with increasing bed depth but decreased with increasing flow rate and influent dye concentration. The column performances were predicted by the application of the bed-depth service time model and Thomas model to the experimental data. The virgin and dye-adsorbed jute fiber was characterized by Fourier transform infrared spectroscopy and scanning electron microscopy analyses. The investigation suggested that jute fiber could be applied as a promising low-cost adsorbent for dye removal. Copyright © 2012 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/app.38222
  • 2013 • 92 Surface grafting of Corchorus olitorius fibre: A green approach for the development of activated bioadsorbent
    Roy, A. and Chakraborty, S. and Kundu, S.P. and Majumder, S.B. and Adhikari, B.
    Carbohydrate Polymers 92 2118-2127 (2013)
    The present work is an endeavor to prepare lignocellulosic biomass based adsorbent, suitable for removal of organic and inorganic pollutants from industrial effluents. Lignocellulosic Corchorus olitorius fibre (jute fibre) surface was grafted with naturally available polyphenol, tannin, preceded by the epoxy-activation of fibre surface with epichlorohydrin under mild condition in an aqueous suspension. The reaction parameters for the modification, viz., concentration of epichlorohydrin and tannin, time, and temperature were optimized. The successful occurrence of surface modification of jute fibre (JF) was characterized and estimated from weight gain percent, elemental analysis, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, scanning electron and atomic force microscopy, and thermogravimetric analysis. An extensive analysis of deconvoluted FTIR spectra using the Voigt model was utilized to ensure the surface grafting. The microbiological susceptibility study revealed high persistency of JF towards biodegradation after efficient grafting with tannin. © 2012 Elsevier Ltd.
    view abstractdoi: 10.1016/j.carbpol.2012.11.039
  • 2013 • 91 Influence of janus particle shape on their interfacial behavior at liquid-liquid interfaces
    Ruhland, T.M. and Gröschel, A.H. and Ballard, N. and Skelhon, T.S. and Walther, A. and Müller, A.H.E. and Bon, S.A.F.
    Langmuir 29 1388-1394 (2013)
    We investigate the self-Assembly behavior of Janus particles with different geometries at a liquid-liquid interface. The Janus particles we focus on are characterized by a phase separation along their major axis into two hemicylinders of different wettability. We present a combination of experimental and simulation data together with detailed studies elucidating the mechanisms governing the adsorption process of Janus spheres, Janus cylinders, and Janus discs. Using the pendant drop technique, we monitor the assembly kinetics following changes in the interfacial tension of nanoparticle adsorption. According to the evolution of the interfacial tension and simulation data, we will specify the characteristics of early to late stages of the Janus particle adsorption and discuss the effect of Janus particle shape and geometry. The adsorption is characterized by three adsorption stages which are based on the different assembly kinetics and different adsorption mechanisms depending on the particle shape. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/la3048642
  • 2013 • 90 Direct electron transfer of Trametes hirsuta laccase adsorbed at unmodified nanoporous gold electrodes
    Salaj-Kosla, U. and Pöller, S. and Schuhmann, W. and Shleev, S. and Magner, E.
    Bioelectrochemistry 91 15-20 (2013)
    The enzyme Trametes hirsuta laccase undergoes direct electron transfer at unmodified nanoporous gold electrodes, displaying a current density of 28μA/cm2. The response indicates that ThLc was immobilised at the surface of the nanopores in a manner which promoted direct electron transfer, in contrast to the absence of a response at unmodified polycrystalline gold electrodes. The bioelectrocatalytic activity of ThLc modified nanoporous gold electrodes was strongly dependent on the presence of halide ions. Fluoride completely inhibited the enzymatic response, whereas in the presence of 150mM Cl-, the current was reduced to 50% of the response in the absence of Cl-. The current increased by 40% when the temperature was increased from 20°C to 37°C. The response is limited by enzymatic and/or enzyme electrode kinetics and is 30% of that observed for ThLc co-immobilised with an osmium redox polymer. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.bioelechem.2012.11.001
  • 2013 • 89 Investigations on diffusion limitations of biocatalyzed reactions in amphiphilic polymer conetworks in organic solvents
    Schoenfeld, I. and Dech, S. and Ryabenky, B. and Daniel, B. and Glowacki, B. and Ladisch, R. and Tiller, J.C.
    Biotechnology and Bioengineering 110 2333-2342 (2013)
    The use of enzymes as biocatalysts in organic media is an important issue in modern white biotechnology. However, their low activity and stability in those media often limits their full-scale application. Amphiphilic polymer conetworks (APCNs) have been shown to greatly activate entrapped enzymes in organic solvents. Since these nanostructured materials are not porous, the bioactivity of the conetworks is strongly limited by diffusion of substrate and product. The present manuscript describes two different APCNs as nanostructured microparticles, which showed greatly increased activities of entrapped enzymes compared to those of the already activating membranes and larger particles. We demonstrated this on the example of APCN particles based on PHEA-l-PDMS loaded with α-Chymotrypsin, which resulted in an up to 28,000-fold higher activity of the enzyme compared to the enzyme powder. Furthermore, lipase from Rhizomucor miehei entrapped in particles based on PHEA-l-PEtOx was tested in n-heptane, chloroform, and substrate. Specific activities in smaller particles were 10- to 100-fold higher in comparison to the native enzyme. The carrier activity of PHEA-l-PEtOx microparticles was tenfold higher with some 25-50-fold lower enzyme content compared to a commercial product. Biotechnol. Bioeng. 2013; 110:2333-2342. © 2013 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/bit.24906
  • 2013 • 88 Trace level adsorption of toxic sulfur compounds, carbon dioxide, and water from methane
    Steuten, B. and Pasel, C. and Luckas, M. and Bathen, D.
    Journal of Chemical and Engineering Data 58 2465-2473 (2013)
    This paper presents breakthrough curves and isotherms of the adsorption of sulfur compounds, carbon dioxide, and water from a carrier gas (methane) on a fixed solid bed at 298 K and 1.3 bar. For the investigation two industrial adsorbents (silica-alumina gel, zeolite 5A) were used. The adsorptives were prepared in trace level concentrations up to 2000 mol-ppm. Common isotherm equations were fitted to the adsorption capacities which were obtained from breakthrough curves by mass balances. Binary systems (one adsorptive in methane) and ternary systems (two adsorptives in methane) are included. Methane is used to duplicate conditions of industrial scale natural gas treatment as far as possible. Though methane is a very weak adsorptive on oxidic adsorbents the reported adsorptive capacities might be slightly lower than pure component loadings accessible from a volumetric or gravimetric method. The adsorption isotherms of the binary systems show distinctly different capacities depending on the polarity of the adsorptive and the structure of the adsorbent. The investigation of the ternary systems reveals significant coadsorption and displacement as well as kinetic effects due to the presence of competing adsorptives. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/je400298r
  • 2013 • 87 Adsorptive removal of sulfurous components from natural gas
    Steuten, B. and Pasel, C. and Luckas, M. and Bathen, D.
    Chemie-Ingenieur-Technik 85 333-343 (2013)
    Prior to the technical use of natural gas, toxic and corrosive components need to be removed. This work provides results from dynamic fixed-bed experiments for the adsorption of sulfurous compounds, CO2 and H 2O from carrier gas (CH4 or N2) on two adsorbents (zeolite 5A, silica-alumina-gel) used in industrial applications. The breakthrough curves were measured at ambient conditions (298 K, 1.3 bar) in a trace level concentration range up to 2000 mol-ppm. Adsorption isotherms were derived using mass balances and a simple linear driving force model was fitted to the curves. Good agreement of experimental data and model calculation was obtained. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201200102
  • 2013 • 86 The constant phase element reveals 2D phase transitions in adsorbate layers at the electrode/electrolyte interfaces
    Tymoczko, J. and Schuhmann, W. and Bandarenka, A.S.
    Electrochemistry Communications 27 42-45 (2013)
    Using one of the most understood and well-characterized electrochemical systems, Pt(111) surface in contact with H2SO4, we provide evidences that specific adsorption, 2D phase transitions in the adsorbate layers and, in general, structural effects in the double layer are largely responsible for the so-called frequency dispersion of the double layer. The results also show promise that parameters of the constant phase element (which is used in impedance spectroscopy to account for the frequency dispersion) obtained as a function of the electrode potential can be reasonably used to detect 2D phase transitions at the electrode/electrolyte interfaces. This would provide a better insight into the interface, increasing the impact of measurements made by electrochemical impedance spectroscopy. © 2012 Elsevier B.V. All rights reserved. All rights reserved.
    view abstractdoi: 10.1016/j.elecom.2012.11.001
  • 2013 • 85 Antibody-imprinted membrane adsorber via two-step surface grafting
    Yin, D. and Ulbricht, M.
    Biomacromolecules 14 4489-4496 (2013)
    In this work, a recently established, novel two-step imprinting strategy combining surface imprinting and scaffold imprinting was applied successfully to prepare a molecularly imprinted polymer (MIP) adsorber for immunoglobulin G (IgG). Track-etched polyethylene terephthalate (PET) membranes with previously introduced aliphatic C-Br groups as initiator on the pore surface were used to prepare first a functional polymer scaffold, grafted poly(methacrylic acid), via surface-initiated atom transfer radical polymerization (SI-ATRP). After template protein (IgG) binding to the scaffold, UV-initiated cross-linking copolymerization of acrylamide and methylenebisacrylamide (MBAA) as second step lead to a grafted MIP hydrogel layer. The influences of the three independent parameters, scaffold chain length by SI-ATRP time, degree of cross-linking of the MIP layer by MBAA content, and grafted MIP layer thickness by UV irradiation time, were studied to optimize protein binding capacity and selectivity. The results were also compared to previously obtained data for lysozyme imprinting using the same method, and significant effects of protein size on imprinting efficiency could be identified. The best IgG MIP membrane adsorber was then used to separate IgG from mixtures with human serum albumin (HSA), demonstrating IgG binding capacities and eluted IgG purities, which were almost independent of the excess of HSA. The results of this study are a significant extension of the scope of molecular imprinting toward large target bionanoparticles. The transfer of the approach from the model PET to other base membranes with higher specific surface area is straightforward, and the resulting affinity materials would, in principle, be suited for "capturing" of an antibody from a complex mixture. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/bm401444y
  • 2013 • 84 Experimental and theoretical investigation of molybdenum carbide and nitride as catalysts for ammonia decomposition
    Zheng, W. and Cotter, T.P. and Kaghazchi, P. and Jacob, T. and Frank, B. and Schlichte, K. and Zhang, W. and Su, D.S. and Schüth, F. and Schlögl, R.
    Journal of the American Chemical Society 135 3458-3464 (2013)
    Constant COx-free H2 production from the catalytic decomposition of ammonia could be achieved over a high-surface-area molybdenum carbide catalyst prepared by a temperature-programmed reduction-carburization method. The fresh and used catalyst was characterized by N2 adsorption/desorption, powder X-ray diffraction, scanning and transmission electron microscopy, and electron energy-loss spectroscopy at different stages. Observed deactivation (in the first 15 h) of the high-surface-area carbide during the reaction was ascribed to considerable reduction of the specific surface area due to nitridation of the carbide under the reaction conditions. Theoretical calculations confirm that the N atoms tend to occupy subsurface sites, leading to the formation of nitride under an NH3 atmosphere. The relatively high rate of reaction (30 mmol/((g of cat.) min)) observed for the catalytic decomposition of NH3 is ascribed to highly energetic sites (twin boundaries, stacking faults, steps, and defects) which are observed in both the molybdenum carbide and nitride samples. The prevalence of such sites in the as-synthesized material results in a much higher H2 production rate in comparison with that for previously reported Mo-based catalysts. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/ja309734u
  • 2012 • 83 Influence of Cs + and Na + on specific adsorption of *oH, *o, and *h at platinum in acidic sulfuric media
    Berkes, B.B. and Inzelt, G. and Schuhmann, W. and Bondarenko, A.S.
    Journal of Physical Chemistry C 116 10995-11003 (2012)
    The influence of Cs + and Na + on the adsorption of *H, *OH, and *O (where * denotes adsorbed species) at polycrystalline Pt in acidic sulfuric media has been investigated. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry, and electrochemical nanogravimetry were used (i) to elucidate the models of the interface between polycrystalline Pt and the electrolytes in a wide range of electrode potentials and (ii) to resolve contributions originating from adsorbed *H, (bi)sulfate, *OH, and *O as well as the cations to the overall interface status. Using impedance analysis it was possible to separate at least two adsorption processes: (bi)sulfate and hydrogen adsorption. The nanogravimetry additionally resolves the contribution from Cs +. Specific adsorption of Cs + at Pt surface significantly affects hydrogen adsorption, while it has almost no effect on the dynamics of SO 4 2- adsorption. Specifically adsorbed alkali cations, however, are desorbed by the onset of *OH(*O) adsorption at the Pt surface. Nevertheless, the cations likely remain in the close proximity to the surface, probably in the second H 2O layer, and largely contribute to the formation of the *OH and *O adsorbed species originating from the surface water. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp300863z
  • 2012 • 82 Fabrication of a CO2-selective membrane by stepwise liquid-phase deposition of an alkylether functionalized pillared-layered metal-organic framework [Cu2L2P]n on a macroporous support
    Bétard, A. and Bux, H. and Henke, S. and Zacher, D. and Caro, J. and Fischer, R.A.
    Microporous and Mesoporous Materials 150 76-82 (2012)
    Metal-organic framework (MOF) membranes were prepared by stepwise deposition of reactants. Two pillared layered MOFs with the general formula [Cu2L2P]n (L = dicarboxylate linker, P = pillaring ligand) were selected. Within this family, fine tuning of adsorption affinity and pore size is possible by variation or functionalization of the L and P linkers. Compound 1 was chosen to be non-polar (L = 1,4- naphtalenedicarboxylate = ndc, P = 1,4-diazabicyclo(2.2.2)octane = dabco); in contrast, compound 2 included a polar linker L with two conformationally flexible ether side chains (L = 2,5-bis(2-methoxyethoxy)-1,4-benzene- dicarboxylate = BME-bdc, P = dabco). The polar functionalization is expected to increase the framework affinity for CO2 compared to CH4. The step-by-step, liquid phase deposition of 1 and 2 resulted in pore-plugging of macroporous ceramic supports. The performances of the two MOF membranes were evaluated in gas separation experiments of equimolar CO2/CH 4 mixtures using a modified Wicke-Kallenbach technique. Anti-Knudsen CO2/CH4 separation factors in the range of ∼4-4.5 were obtained for the membrane consisting of the polar 2, whereas the separation of the membrane formed from the non-polar 1 was found to be Knudsen-like. © 2011 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.micromeso.2011.09.006
  • 2012 • 81 Electron transport at surfaces and interfaces
    Bobisch, C.A. and Möller, R.
    Chimia 66 23-30 (2012)
    Here we present two techniques which give insight on transport phenomena with atomic resolution. Ballistic electron emission microscopy is used to study the ballistic transport through layered heterogeneous systems. The measured ballistic fraction of the tunneling current provides information about lossless transport channels through metallic layers and organic adsorbates. The transport characteristics of Bi(111)/Si Schottky devices and the influence of the organic adsorbates perylene tetracaboxylic dianhydride acid and C 60 on the ballistic current are discussed. Scanning tunneling potentiometry gives access to the lateral transport along a surface, thus scattering processes within two-dimensional electron systems for the Bi(111) surface and the Si(111)(√3×√3)-Ag surface could be visualized. © Schweizerische Chemische Gesellschaft.
    view abstractdoi: 10.2533/chimia.2012.23
  • 2012 • 80 Adsorption of nanoparticles at the solid-liquid interface
    Brenner, T. and Paulus, M. and Schroer, M.A. and Tiemeyer, S. and Sternemann, C. and Möller, J. and Tolan, M. and Degen, P. and Rehage, H.
    Journal of Colloid and Interface Science 374 287-290 (2012)
    The adsorption of differently charged nanoparticles at liquid-solid interfaces was investigated by in situ X-ray reflectivity measurements. The layer formation of positively charged maghemite (γ-Fe 2O 3) nanoparticles at the aqueous solution-SiO 2 interface was observed while negatively charged gold nanoparticles show no adsorption at this interface. Thus, the electrostatic interaction between the particles and the charged surface was determined as the driving force for the adsorption process. The data analysis shows that a logarithmic particle size distribution describes the density profile of the thin adsorbed maghemite layer. The size distribution in the nanoparticle solution determined by small angle X-ray scattering shows an average particle size which is similar to that found for the adsorbed film. The formed magehemite film exhibits a rather high stability. © 2012 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcis.2012.02.010
  • 2012 • 79 Impact of the nanoparticle-protein corona on colloidal stability and protein structure
    Gebauer, J.S. and Malissek, M. and Simon, S. and Knauer, S.K. and Maskos, M. and Stauber, R.H. and Peukert, W. and Treuel, L.
    Langmuir 28 9673-9679 (2012)
    In biological fluids, proteins may associate with nanoparticles (NPs), leading to the formation of a so-called "protein corona" largely defining the biological identity of the particle. Here, we present a novel approach to assess apparent binding affinities for the adsorption/desorption of proteins to silver NPs based on the impact of the corona formation on the agglomeration kinetics of the colloid. Affinities derived from circular dichroism measurements complement these results, simultaneously elucidating structural changes in the adsorbed protein. Employing human serum albumin as a model, apparent affinities in the nanomolar regime resulted from both approaches. Collectively, our findings now allow discrimination between the formation of protein mono- and multilayers on NP surfaces. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/la301104a
  • 2012 • 78 Adsorption of aromatic trace compounds from organic solvents on activated carbons-experimental results and modeling of adsorption equilibria
    Gräf, T. and Pasel, C. and Luckas, M. and Bathen, D.
    Adsorption 18 127-141 (2012)
    Liquid phase adsorption is an important process for the removal of trace compounds from liquid matrices. Until today, research on liquid phase adsorption is less substantial than work on other thermal separation processes. The description of relevant mechanisms and interactions is difficult mainly because of lacking experimental data. This paper presents extensive isotherm measurements for the adsorption of organic trace compounds from organic solvents on activated carbons. A systematic variation of molecular structure of adsorptives and solvents enabled the identification of main structural factors dominating adsorption in these systems. The factors are polarity, extension and density of π electrons and sterical complexity. An analysis of the measured isotherms revealed incremental effects of functional groups and structural elements being characteristic for the adsorption capacities on activated carbons. Three consecutive empirical prediction models of adsorption equilibria are developed and compared. The empirical Freundlich equation appeared to be best suited for fitting the experimental data. The models apply an incremental concept permitting the calculation of adsorption isotherms on the basis of the structural increments of solvent and adsorptive molecules. The three models have a different extent of underlying data, a different number of parameters and a different range of application. The experimental data are predicted with satisfying accuracy for many engineering applications. The most sophisticated model has the most extensive range of application and manages on the smallest number of parameters. © 2012 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s10450-012-9388-0
  • 2012 • 77 Detection of DNA hybridization using electrochemical impedance spectroscopy and surface enhanced Raman scattering
    Grützke, S. and Abdali, S. and Schuhmann, W. and Gebala, M.
    Electrochemistry Communications 19 59-62 (2012)
    The formation of double-stranded DNA (dsDNA) at gold electrodes decorated with a monolayer of gold nanoparticles bound through a self-assembled dithiol monolayer is detected via specific intercalation of proflavine. Hybridization as well as sequential built-up of the electrode architecture is monitored using Faradaic electrochemical impedance spectroscopy (EIS) as well as surface enhanced Raman scattering (SERS). The adsorption of secondary gold nanoparticles allow for amplified detection of the dsDNA integrated intercalator in a vertical gap mode configuration. The experimental design thus allows probing presence of the intercalator inside the dsDNA. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elecom.2012.03.026
  • 2012 • 76 Subsurface influence on the structure of protein adsorbates as revealed by in situ X-ray reflectivity
    Hähl, H. and Evers, F. and Grandthyll, S. and Paulus, M. and Sternemann, C. and Loskill, P. and Lessel, M. and Hüsecken, A.K. and Brenner, T. and Tolan, M. and Jacobs, K.
    Langmuir 28 7747-7756 (2012)
    The adsorption process of proteins to surfaces is governed by the mutual interactions among proteins, the solution, and the substrate. Interactions arising from the substrate are usually attributed to the uppermost atomic layer. This actual surface defines the surface chemistry and hence steric and electrostatic interactions. For a comprehensive understanding, however, the interactions arising from the bulk material also have to be considered. Our protein adsorption experiments with globular proteins (α-amylase, bovine serum albumin, and lysozyme) clearly reveal the influence of the subsurface material via van der Waals forces. Here, a set of functionalized silicon wafers enables a distinction between the effects of surface chemistry and the subsurface composition of the substrate. Whereas the surface chemistry controls whether the individual proteins are denatured, the strength of the van der Waals forces affects the final layer density and hence the adsorbed amount of proteins. The results imply that van der Waals forces mainly influence surface processes, which govern the structure formation of the protein adsorbates, such as surface diffusion and spreading. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/la300850g
  • 2012 • 75 Directing the breathing behavior of pillared-layered metal-organic frameworks via a systematic library of functionalized linkers bearing flexible substituents
    Henke, S. and Schneemann, A. and Wütscher, A. and Fischer, R.A.
    Journal of the American Chemical Society 134 9464-9474 (2012)
    Flexible metal-organic frameworks (MOFs), also referred to as soft porous crystals (SPCs), show reversible structural transitions dependent on the nature and quantity of adsorbed guest molecules. In recent studies it has been reported that covalent functionalization of the organic linker can influence or even integrate framework flexibility ("breathing") in MOFs. However, rational fine-tuning of such responsive properties is very desirable but challenging as well. Here we present a powerful approach for the targeted manipulation of responsiveness and framework flexibility of an important family of pillared-layered MOFs based on the parent structure [Zn 2(bdc) 2(dabco)] n (bdc = 1,4-benzenedicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]octane). A library of functionalized bdc-type linkers (fu-bdc), which bear additional dangling side groups at different positions of the benzene core (alkoxy groups of varying chain length with diverse functionalities and polarity), was generated. Synthesis of the materials [Zn 2(fu-bdc) 2(dabco)] n yields the respective collection of highly responsive MOFs. The parent MOF is only weakly flexible; however, the substituted frameworks of [Zn 2(fu-bdc) 2(dabco)] n contract drastically upon guest removal and expand again upon adsorption of DMF (N,N-dimethylformamide), EtOH, or CO 2, etc., while N 2 is hardly adsorbed and does not open the narrow-pored form. These "breathing" dynamics are attributed to the dangling side chains that act as immobilized "guests", which interact with mobile guest molecules as well as with themselves and with the framework backbone. The structural details of the guest-free, contracted form and the gas sorption behavior (phase transition pressure, hysteresis loop) are highly dependent on the nature of the substituent at the linker and can therefore be adjusted using our approach. Combining our library of functionalized linkers with the concept of mixed-component MOFs (solid solutions) offers very rich additional dimensions of tailoring the structural dynamics and responsiveness. Implementation of two differently functionalized linkers in varying ratios yields multicomponent single-phased [Zn 2(fu-bdc ) 2x(fu- bdc″) 2-2x(dabco)] n MOFs (0 &lt; x &lt; 1) of increased inherent complexity, which feature a non-linear dependence of their gas sorption properties on the applied ratio of components. Hence, the responsive behavior of such pillared-layered MOFs can be extensively tuned via an intelligent combination of functionalized linkers. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/ja302991b
  • 2012 • 74 A switch based on self-assembled thymine
    Kalkan, F. and Mehlhorn, M. and Morgenstern, K.
    Journal of Physics-condensed Matter 24 394010 (2012)
    The DNA base thymine is deposited at 100 K on Cu(111) and investigated and manipulated by low-temperature scanning tunneling microscopy at 5 K. At submonolayer coverage paired rows are observed. At monolayer coverage a hexagonal commensurate self-assembled layer with the methyl group pointing away from the surface forms. A reversible local manipulation of molecules within the self-assembled layer is demonstrated. This manipulation is interpreted as an out-of-plane relaxation of molecules within the layer induced by the change of the adsorption geometry of individual molecules between two meta-stable orientations. A positive field of 2-4 V leads to this local change in the molecular arrangement, while a field larger than 4 V restores the original geometry.
    view abstractdoi: 10.1088/0953-8984/24/39/394010
  • 2012 • 73 Interfacial interaction driven CO oxidation: Nanostructured Ce 1-xLa xO 2-δ/TiO 2 solid solutions
    Katta, L. and Reddy, B.M. and Muhler, M. and Grünert, W.
    Catalysis Science and Technology 2 745-753 (2012)
    Titania supported ceria-lanthana solid solutions (Ce xLa 1-xO 2-δ/TiO 2; CLT) have been synthesized by a facile and economical route. Existence of synergism between ceria-lanthana (CL) solid solutions and titania-anatase phase, which leads to decrease in the crystallite size, retarded titania phase transformation, and improved redox properties, has been thoroughly investigated by various techniques, namely, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), UV-visible diffuse reflectance spectroscopy (UV-vis DRS), Raman spectroscopy (UV-RS and Vis-RS), BET surface area analysis, and temperature programmed reduction (TPR). Two key observations made from the whole exercise were (i) mutual interaction of Ce and Ti ions could impose typical Ce-O-Ti modes at the interfacial region and (ii) the La 3+ ion as a dopant provokes a large number of oxygen vacancies via a charge compensation mechanism. The promising role of these factors in the CO oxidation (one of the most formidable challenges) has been comprehensively described. The observed enhanced activity for the CLT sample is primarily attributed to an apparent specific orientation of the active component over the support, which is endorsed by the interfacial interaction. This specific mode could facilitate the CO adsorption with simultaneous bulk oxygen diffusion for more consumption and in turn better activity. © 2012 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c2cy00449f
  • 2012 • 72 Is there sp 3-bound H on epitaxial graphene? Evidence for adsorption on both sides of the sheet
    Kim, H. and Balgar, T. and Hasselbrink, E.
    Chemical Physics Letters 546 12-17 (2012)
    IR-vis sum-frequency generation spectroscopy is used to study the stretching vibration of hydrogen chemically bound to a monolayer graphene sheet prepared on an Ir (1 1 1) substrate. A line characteristic for sp3 bound hydrogen is observed when a mixture of H and D is dosed. The intensity per surface H oscillator is largest when H is strongly diluted in D. This is interpreted to indicate that graphane is formed by concurrent attachment of hydrogen to graphene from the gas phase and from intercalated hydrogen. In this case isotope mixing is a prerequisite for SFG activity. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cplett.2012.07.049
  • 2012 • 71 The necessity for the coating of perfluorodecalin-filled poly(lactide-co-glycolide) microcapsules in the presence of physiological cholate concentrations: Tetronic-908 as an exemplary polymeric surfactant
    Kirsch, M. and Bramey, T. and Waack, I.N. and Petrat, F. and Mayer, C. and De Groot, H.
    Journal of Microencapsulation 29 30-38 (2012)
    Recently, we demonstrated that biodegradable poly(lactide-co-glycolide) (PLGA) micro- and nanocapsules with a liquid content of perfluorodecalin are principally useful for the development of artificial oxygen carriers. In order to solve a decisive and well-known problem with PLGA microcapsules, i.e. the spontaneous agglomeration of the capsules after depletion of the emulsifying agent (i.e. cholate), coating with the ABA block copolymer, Tetronic-908 was studied. After Tetronic-908 treatment at concentrations that were harmless to cultured cells, the clustering of the microcapsules was prevented, the adsorption of opsonins was decreased and the attachment to cells was inhibited, but the oxygen transport capacity of PLGA microcapsules was even increased. The present data clearly show that perfluorodecalin-filled PLGA microcapsules must be coated before decreasing the emulsifying agent cholate to physiological concentrations, in order to develop a solution that has the capabilities to function as a potential artificial oxygen carrier suspension. © 2012 Informa UK Ltd All rights reserved.
    view abstractdoi: 10.3109/02652048.2011.629743
  • 2012 • 70 Freeze-dried cationic calcium phosphate nanorods as versatile carriers of nucleic acids (DNA, siRNA)
    Klesing, J. and Chernousova, S. and Epple, M.
    Journal of Materials Chemistry 22 199-204 (2012)
    Functionalized calcium phosphate (hydroxyapatite) nanorods were freeze-dried in the presence of the cryoprotectant trehalose, giving a storable and easily redispersible system which can adsorb nucleic acids for transfection and gene silencing. The nanorods were first surface-functionalized with a layer of polyethyleneimine (PEI), purified by ultracentrifugation and redispersion, and freeze-dried in the presence of trehalose. The nanorods can be easily redispersed in water. The concentration-dependent adsorption of nucleic acids (DNA and siRNA) onto the surface of the redispersed cationic nanorods was measured by dynamic light scattering (particle diameter and zeta potential). The transfection efficiency on epithelial cells (HeLa) and osteoblasts (MG-63) was systematically determined for increasing amounts of added nucleic acid, up to a charge reversal by the anionic nucleic acids. Both transfection and gene silencing efficiency increased with increasing amount of nucleic acid, but went through minimum around the point of zero charge where the particles agglomerated. The application of an additional outer layer of polyethyleneimine around the hydroxyapatite/PEI/nucleic acid-nanoparticles reversed the charge back to positive, resulting in even higher transfection rates with almost complete cell viability.
    view abstractdoi: 10.1039/c1jm13502c
  • 2012 • 69 Catalytic role of gold nanoparticle in GaAs nanowire growth: A density functional theory study
    Kratzer, P. and Sakong, S. and Pankoke, V.
    Nano Letters 12 943-948 (2012)
    The energetics of Ga, As, and GaAs species on the Au(111) surface (employed as a model for Au nanoparticles) is investigated by means of density functional calculations. Apart from formation of the compound Au 7Ga 2, Ga is found to form a surface alloy with gold with comparable ΔH ∼ -0.5 eV for both processes. Dissociative adsorption of As 2 is found to be exothermic by more than 2 eV on both clean Au(111) and AuGa surface alloys. The As-Ga species formed by reaction of As with the surface alloy is sufficiently stable to cover the surface of an Au particle in vacuo in contact with a GaAs substrate. The results of the calculations are interpreted in the context of Au-catalyzed growth of GaAs nanowires. We argue that arsenic is supplied to the growth zone of the nanowire mainly by impingement of molecules on the gold particle and identify a regime of temperatures and As 2 partial pressures suitable for Au-catalyzed nanowire growth in molecular beam epitaxy. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/nl204004p
  • 2012 • 68 Nanostructured supported palladium catalysts - Non-oxidative methane coupling
    Moya, S.F. and Martins, R.L. and Ota, A. and Kunkes, E.L. and Behrens, M. and Schmal, M.
    Applied Catalysis A: General 411-412 105-113 (2012)
    The Pd on α-Al2O3 catalysts with Pd particles in the low nanometer range have been prepared by a sonochemical reduction and a colloidal method, respectively. The two catalysts differ in their particle size, the widths of their particle size distributions and the amount of carbon incorporation in the Pd lattice. The adsorptive properties of the Pd/Al 2O3 samples are different as a result of the different preparation methods. The methane adsorption capacity of that sample with smaller particles is lower than that of the catalyst with larger particles and the energy of activation is nearly doubled. DRIFTS and TPD results of CO adsorption supported by transmission electron microscopy data indicate that the PdSON catalyst with smaller and more homogeneous particles than PdCOL is highly dispersed which influences the coupling-hydrogenolysis process. The catalytic activity evidenced the formation of different adspecies during methane coupling and chemisorption on both catalysts. During the hydrogenation the carbon adspecies formed mainly methane at low adsorption temperatures. The significant amount of adsorbed methane at 773 K is governed by the highly active coordination unsaturated sites at the surface. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apcata.2011.10.025
  • 2012 • 67 Hydrogen adsorption and site-selective reduction of the Fe 3O 4(001) surface: Insights from first principles
    Mulakaluri, N. and Pentcheva, R.
    Journal of Physical Chemistry C 116 16447-16453 (2012)
    Density functional theory calculations including an on-site Hubbard term are used to explore hydrogen adsorption on the surface of Fe 3O 4(001). The adsorption energy exhibits a minimum for two hydrogen atoms per (√2 × √2)R45° surface unit cell and gets less favorable with increasing hydrogen coverage due to OH-OH repulsion. Terminations with two and four hydrogen atoms per surface unit cell are stable for moderate to high partial pressures of O and H. The strong tilt of the OH bond parallel to the surface facilitates hydrogen bonding to neighboring oxygen and hopping of the protons between surface oxygen sites. Furthermore, the formation of surface OH groups leads to a monotonic reduction of work function with increasing H coverage. The analysis of the electronic properties reveals selective switching of neighboring surface and subsurface Fe from Fe 3+ to Fe 2+ upon hydrogen adsorption. This provides a promising way to tune the catalytic activity of the Fe 3O 4(001) surface. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp302259d
  • 2012 • 66 Probing the mechanism of low-temperature CO oxidation on Au/ZnO catalysts by vibrational spectroscopy
    Noei, H. and Birkner, A. and Merz, K. and Muhler, M. and Wang, Y.
    Journal of Physical Chemistry C 116 11181-11188 (2012)
    Adsorption and oxidation of CO on Au/ZnO catalysts were studied by Fourier transform infrared (FTIR) spectroscopy using a novel ultra-high-vacuum (UHV) system. The high-quality UHV-FTIRS data provide detailed insight into the catalytic mechanism of low-temperature CO oxidation on differently pretreated Au/ZnO catalysts. For the samples without O 2 pretreatment, negatively charged Au nanoparticles are identified which exhibit high reactivity to CO oxidation at 110 K, yielding CO 2 as well as carbonate species bound to various ZnO facets. O 2 pretreatment leads to formation of neutral Au nanoparticles where CO is activated on the low-coordinated Au sites at the interface. Activation of impinging O 2 occurs at the Au/ZnO interface and is promoted by preadsorbed CO forming an OC-O 2 intermediate complex, accompanied by charge transfer from Au/ZnO substrate to O 2. The CO molecules adsorbed on ZnO serve as a reservoir for reactants and are mobile enough at 110 K to reach the Au/ZnO interface where they react with activated oxygen yielding CO 2. Different carbonate species are further produced via interaction of formed CO 2 with surface oxygen atoms on ZnO. It was found that the active interface sites are slowly blocked at 110 K by the inert carbonate species, thus causing a gradual decrease of the catalytic activity. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/jp302723r
  • 2012 • 65 Density functional theory study of water adsorption on FeOOH surfaces
    Otte, K. and Schmahl, W.W. and Pentcheva, R.
    Surface Science 606 1623-1632 (2012)
    Using density functional theory (DFT) calculations with an on-site Coulomb repulsion term, we study the composition, stability, and electronic properties of the most common FeOOH surfaces goethite(101), akaganeite(100), and lepidocrocite(010), and their interaction with water. Despite the differences in surface structure, the trends in surface stability of these FeOOH polymorphs exhibit remarkable similarities. We find that the reactivity and the binding configuration of adsorbates depend strongly on the coordination of surface iron: at the fourfold coordinated Fe2 site water is chemisorbed, whereas at the fivefold coordinated Fe1 water is only loosely bound with hydrogen pointing towards the surface. Our results show that the oxidation state of surface iron can be controlled by the surface termination where ferryl (Fe 4 +) species emerge for oxygen terminated surfaces and ferrous iron (Fe 2 +) at iron and water terminations leading to a reduced band gap. In contrast, the fully hydroxylated surfaces, identified as stable surface configurations at standard conditions from the surface phase diagram, show electronic properties and band gaps closest to bulk FeOOH with ferric surface iron (Fe 3 +). Only in the case of goethite(101), a termination with mixed surface hydroxyl and aquo groups is stabilized. © 2012 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2012.07.009
  • 2012 • 64 Adsorptive water removal from primary alcohols and acetic acid esters in the ppm-region
    Pahl, C. and Pasel, C. and Luckas, M. and Bathen, D.
    Journal of Chemical and Engineering Data 57 2465-2471 (2012)
    Dry organic solvents are important for many industrial sectors. Adsorptive water removal is one technique to obtain highly pure solvents. However, in-depth knowledge of the parameters influencing adsorption behavior is still fragmentary. This paper presents a systematic investigation of water adsorption from alcohols (C1 to C6) and acetic acid esters (methyl acetate to n-butyl acetate) of different chain lengths. Zeolites of types 3A and 4A are used as adsorbents. The impact of size exclusion on adsorption properties is analyzed. The water adsorption isotherms on zeolite 3A from solvents with a large critical molecular diameter are similar to the water vapor isotherm as expected from literature data. In case of smaller solvent molecules (methanol, ethanol, 1-propanol) a significantly lower water adsorption capacity is found on zeolite 3A. In case of all solvents on zeolite 4A water adsorption is lower than water vapor adsorption even if the estimated molecular diameter is larger than the reported window aperture of the zeolite cage. It is discussed to what extent water and solvent are capable to coadsorb and compete for adsorption sites. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/je3005694
  • 2012 • 63 Adsorption of anionic-azo dye from aqueous solution by lignocellulose- biomass jute fiber: Equilibrium, kinetics, and thermodynamics study
    Roy, A. and Chakraborty, S. and Kundu, S.P. and Adhikari, B. and Majumder, S.B.
    Industrial and Engineering Chemistry Research 51 12095-12106 (2012)
    The present investigation describes the evaluation of feasibility of lignocellulosic-biomass jute fiber (JF) toward adsorptive removal of anionic-azo dye from aqueous solution. Batch studies illustrated that dye uptake was highly dependent on different process variables, pH, initial dye concentration of solution, adsorbent dosage, and temperature. Further, an attempt has been taken to correlate these process variables with dye absorption and was optimized through a full-factorial central composite design (CCD) in response surface methodology (RSM). Maximum adsorption capacity (29.697 mg/g) under optimum conditions of variables (pH 3.91, adsorbent dose 2.04 g/L, adsorbate concentration 244.05 mg/L, and temperature 30 °C), as predicted by RSM, was found to be very close to the experimentally determined value (28.940 mg/g). Exothermic and spontaneous nature of adsorption was revealed from thermodynamic study. Equilibrium adsorption data were highly consistent with Langmuir isotherm yielding R 2 = 0.999. Kinetic studies revealed that adsorption followed pseudo second-order model regarding the intraparticle diffusion. Activation parameters for the adsorption process were computed using Arrhenius and Eyring equations. Maximum desorption efficiency of spent adsorbent was achieved using sodium hydroxide solution (0.1 M). © 2012 American Chemical Society.
    view abstractdoi: 10.1021/ie301708e
  • 2012 • 62 Multilayered high surface area "brick and mortar" mesoporous titania films as efficient anodes in dye-sensitized solar cells
    Szeifert, J.M. and Fattakhova-Rohlfing, D. and Rathouský, J. and Bein, T.
    Chemistry of Materials 24 659-663 (2012)
    The "brick and mortar" approach is employed to synthesize thick surfactant-templated mesoporous titanium dioxide films of up to 10 μm thickness using multilayer deposition. The films exhibit very high surface areas scaling linearly with the thickness, and roughness factors of up to 1600 cm 2/cm 2 can be reached. For the first time, surfactant-derived mesoporous titanium dioxide films of such a large thickness and surface area can be prepared without serious cracking, delamination, or deterioration of the porous structure. The mesopores are rather large (12 nm), and stacking many layers does not affect their size or accessibility, which is shown by krypton and dye adsorption experiments. Applied in dye-sensitized solar cells, the films feature a high power conversion efficiency of over 7% already at thicknesses below 4 μm due to their high surface area and dye adsorption. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/cm202218w
  • 2012 • 61 Quantifying the influence of polymer coatings on the serum albumin corona formation around silver and gold nanoparticles
    Treuel, L. and Malissek, M. and Grass, S. and Diendorf, J. and Mahl, D. and Meyer-Zaika, W. and Epple, M.
    Journal of Nanoparticle Research 14 (2012)
    When nanoparticles (NPs) come into contact with biological fluids, proteins, and other biomolecules interact with their surface. Upon exposure to biological fluids a layer of proteins adsorbs onto their surface, the so-called protein corona, and interactions of biological systems with NPs are therefore mediated by this corona. Here, interactions of serum albumin with silver and gold NPs were quantitatively investigated using circular dichroism spectroscopy. Moreover, surface enhanced Raman spectroscopy was used for further elucidation of protein binding to silver surfaces. The decisive role of poly(vinylpyrrolidone), coatings on the protein adsorption was quantitatively described for the first time and the influential role of the polymer coatings is discussed. Research in nanotoxicology may benefit from such molecular scale data as well as scientific approaches seeking to improve nanomedical applications by using a wide range of polymer surface coatings to optimize biological transport and medical action of NPs. © Springer Science+Business Media B.V. 2012.
    view abstractdoi: 10.1007/s11051-012-1102-3
  • 2012 • 60 How citrate ligands affect nanoparticle adsorption to microparticle supports
    Wagener, P. and Schwenke, A. and Barcikowski, S.
    Langmuir 28 6132-6140 (2012)
    Residual ligands from colloidal synthesis of nanoparticles influence adsorption of nanoparticles to supports and may complicate fabrication of nanoparticle-decorated microparticles. In this work, we studied the adsorption of completely ligand-free metal nanoparticles and controlled ligand-functionalized nanoparticles to chemically inert microparticle supports. Adsorption of ligand-free silver nanoparticles to barium sulfate microparticle supports is a quantitative, nonreversible process following Freundlich adsorption isotherm. However, adsorption efficiency is very sensitive to ligand concentration applied during laser-based synthesis of silver nanoparticles: exceeding a specific threshold concentration of 50 μmol/L citrate equal to a nanoparticle ligand surface coverage of about 50%, results in an almost complete prevention of nanoparticle adsorption because of electrosteric repulsion by ligand shell. Laser-based synthesis of nanoparticle-decorated microparticles is demonstrated with a variety of metal nanoparticles (Ag, Au, Pt, Fe) and supporting microparticles (calcium phosphate, titanium dioxide, barium sulfate) with application potential in heterogeneous catalysis or biomedicine where ligand control offers extra value, like enhanced catalytic activity or biocompatibility. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/la204839m
  • 2012 • 59 Novel membrane adsorbers with grafted zwitterionic polymers synthesized by surface-initiated ATRP and their salt-modulated permeability and protein binding properties
    Yang, Q. and Ulbricht, M.
    Chemistry of Materials 24 2943-2951 (2012)
    A novel zwitterionic polymer functionalized porous membrane adsorber was obtained by grafting poly(N,N-dimethyl-N-methacryloyloxyethyl-N-(3-sulfopropyl) ammonium betaine) (polySPE) to poly(ethylene terephthalate) (PET) track-etched membrane surface via surface-initiated atom transfer radical polymerization (SI-ATRP). The ATRP conditions were optimized, the thus established grafting was well-controlled, and the degree of grafting could be adjusted. Functionalized membranes with a degree of grafting of about 3.5 μg/cm 2 relative to the specific surface area showed almost zero values of zeta potential estimated from the trans-membrane streaming potential measurements. Typical "anti-polyelectrolyte" effect was observed for the polySPE grafted membranes. Flux through the membrane was reduced by adding chaotropic chloride and perchlorate salts to the solution which extended the polySPE chains grafted on the membrane pore wall. Perchlorate salt exhibited much stronger effect on polySPE chain conformation than chloride salt and for a membrane with a degree of grafting of 2.7 μg/cm 2, even 2 mM KClO 4 could extend the thickness of the polymer layer to more than two times (∼43 nm) of that in pure water (∼20 nm). On the contrary, small amounts of kosmotropic ions (10 mM SO 4 2-) further "salted out" the polySPE chains and led to a slightly increased flux. PolySPE grafted PET membranes with different degree of grafting were then used as membrane adsorber for protein binding. Human IgG was used as model protein and the binding capacity was evaluated under both static (no convective flow through the membrane) and dynamic conditions (flow-through conditions). Static adsorption experiments showed that IgG could be loaded to the membrane at medium salt concentration and 85-95% of bound protein could be eluted at either low (zero) or very high salt concentrations. Dynamic flow-through experiments then revealed the influences of salt concentration and salt type on IgG binding. Effects of two chaotropic salts, NaCl and NaClO 4, were evaluated. Slight but not negligible binding of IgG from pure water was suppressed by adding NaCl. IgG binding was then increased in the NaCl concentration range of 100-500 mM and reached a maximum binding capacity value at about 500 mM. Further increase of NaCl concentration led to a decreased binding again. KClO 4 showed similar effects onto IgG binding, but this salt functions in a much lower and much narrower concentration range. All results with respect to grafted layer swelling and protein binding followed the empirical Hofmeister series. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/cm301116p
  • 2012 • 58 Sorption profile of Hg(II) onto mixed phase of copper sulphide and copper sulphate
    Yoong, Y.L.A. and Yap, P.L. and Kutty, M.G. and Timpe, O. and Behrens, M. and Hamid, S.B.A. and Schlögl, R.
    Advanced Materials Research 356-360 537-546 (2012)
    The use of surface oxidized covellite (CuS), namely mixed phase copper sulphide (CuS and CuSO4) was studied for the removal of mercury from aqueous solution under the effect of various reaction parameters (pH, time, Hg(II) concentration). From batch sorption studies, the equilibrium data revealed that the sorption behaviour of Hg(II) onto mixed phase copper sulphide follows well with Langmuir isotherm and the maximum sorption capacity (Q max) determined π 400mg Hg(II) /g of sorbent. Meanwhile, all the unreacted and reacted mixed phase copper sulphides were also characterized by Powder XRD, SEM and XPS techniques. The results indicated that the sorption of Hg(II) onto mixed phase copper sulphide occurs initially through the dissolution of surface oxidized CuSO4 layer. After that, the surface complexation product formed and sorbed onto the surface of CuS. These outcomes suggest the potential ability of CuS in removing Hg(II) even if the CuS layer is being surrounded by oxidized layer of CuSO4. © (2012) Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/www.scientific.net/AMR.356-360.537
  • 2012 • 57 Role of Water in the Chlorine Evolution Reaction at RuO 2-Based Electrodesa-Understanding Electrocatalysis as a Resonance Phenomenon
    Zeradjanin, A.R. and Menzel, N. and Strasser, P. and Schuhmann, W.
    ChemSusChem 5 1897-1904 (2012)
    The reaction path of the Cl 2 evolution reaction (CER) was investigated by combining electrochemical and spectroscopic methods. It is shown that oxidation and reconstruction of the catalyst surface during CER is a consequence of the interaction between RuO 2 and water. The state of the RuO 2 surface during the electrochemical reaction was analyzed in situ by using Raman spectroscopy to monitor vibrations of the crystal lattice of RuO 2 and changes in the surface concentration of the adsorbed species as a function of the electrode potential. The role of the solvent was recognized as being crucial in the formation of an oxygen-containing hydrophilic layer, which is a key prerequisite for electrocatalytic Cl 2 formation. Water (more precisely the OH adlayer) is understood not just as a medium that allows adsorption of intermediates, but also as an integral part of the intermediate formed during the electrochemical reaction. New insights into the general understanding of electrocatalysis were obtained by utilizing the vibration frequencies of the crystal lattice as a dynamic catalytic descriptor instead of thermodynamic descriptors, such as the adsorption energy of intermediates. Interpretation of the derived "volcano" curve suggests that electrocatalysis is governed by a resonance phenomenon. Water powered! The reaction path of the Cl 2 evolution reaction (CER) is investigated by combining electrochemical and spectroscopic methods. Oxidation and reconstruction of the catalyst surface during CER is a consequence of the interaction between RuO 2 and water. Interpretation of the derived volcano curve suggests that electrocatalysis is governed by a resonance phenomenon (see picture). © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201200193
  • 2011 • 56 Photo-chemical surface modification for the control of protein adsorption on textile substrates
    Bahners, T. and Klingelhöller, K. and Ulbricht, M. and Wego, A. and Schollmeyer, E.
    Journal of Adhesion Science and Technology 25 2219-2238 (2011)
    Cell growth on fiber surfaces is an important aspect of many applications of technical textiles. The need to prevent clogging in artificial blood vessels or in textiles used for blood or water filtration as well as the anti-fouling properties of outdoor technical textiles are examples in this context. Since the adsorption of proteins forms the initial step of cell growth, a promising way to avoid biofouling is to prohibit protein adsorption by means of a suitable, permanent and non-toxic surface functionalization. Today, the deposition of poly(ethylene glycol)s (PEGs) is a well-known approach to decrease non-specific protein adsorption. In this work, a photo-chemical method to graft or cross-link PEGs on fiber surfaces was studied. Monomethacrylated PEG300MA and PEG2080MA as well as dimethacrylated PEG400DMA and PEG600DMA were considered, the numbers indicate average molar mass in g/mol. Textile fabrics made of poly(ethylene terephthalate) (PET) were impregnated with solutions of the PEGs and irradiated using either a KrCl* or a XeCl* excimer lamp (emission wavelengths 222 or 308 nm, respectively). Surface properties of the treated textiles were characterized as a function of process conditions using various surface sensitive analyses. UV cross-linking of PEG400DMA resulted in the deposition of a thick layer which effectively masked the texture of the fabric and its pore system. Much less coverage was observed in case of monomethacrylated PEGMAs, with a significant reduction in drop penetration time already after deposition of a marginal layer (less than 0.01 mg/mg). Highest reductions in adsorption of bovine serum albumin (BSA) were observed for samples prepared using PEG300MA or PEG400DMA under conditions where also the drop penetration time was at its minimum. The longer chain PEG2080MA was less effective. All results show clearly that the protein adsorption tendency can be significantly reduced by choice of suitable combinations of PEGylated monomer and UV irradiation conditions. © 2011 Koninklijke Brill NV, Leiden.
    view abstractdoi: 10.1163/016942411X574925
  • 2011 • 55 Light driven reactions of single physisorbed azobenzenes
    Bazarnik, M. and Henzl, J. and Czajka, R. and Morgenstern, K.
    Chemical Communications 47 7764-7766 (2011)
    We present a successful attempt of decoupling a dye molecule from a metallic surface via physisorption for enabling direct photoisomerization. Effective switching between the isomers is possible by exposure to UV light via the rotation pathway. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c1cc11578b
  • 2011 • 54 Simultaneous acquisition of impedance and gravimetric data in a cyclic potential scan for the characterization of nonstationary electrode/electrolyte interfaces
    Berkes, B.B. and Maljusch, A. and Schuhmann, W. and Bondarenko, A.S.
    Journal of Physical Chemistry C 115 9122-9130 (2011)
    Simultaneous acquisition of electrochemical impedance spectroscopy and quartz crystal microbalance (EIS-EQCM) data in cyclic electrode potential scans was used to characterize nonstationary underpotential deposition (UPD) of atomic layers of Ag on Au and Cu on Pt. Both EIS and EQCM data sets complemented each other in the elucidation of interface models and the investigation of different aspects of the interfacial dynamics. EIS-EQCM provided an opportunity to monitor coadsorption and competitive adsorption of anions during the Ag and Cu UPD using (i) the electrode mass change, (ii) adsorption capacitances, and (iii) double-layer capacitances. Kinetic information is available in the EIS-EQCM through the charge transfer resistances and apparent rate coefficients. The latter expresses the rate of UPD into the partially covered electrode surface. The apparent rate coefficients for the Ag UPD were determined to vary from 0.15 to 0.45 cm/s which is between the standard constant rates k0 of Ag bulk deposition on Ag reported previously for different Ag surfaces. Cu UPD on Pt and Ag UPD on Au contributed differently into a resonance resistance ?R(E) available from the EQCM data sets. Spontaneous surface alloying between Ag and Au during the Ag UPD continuously increased the ?R, while the Cu overlayer formation on Pt as well as experiments without Ag+ and Cu 2+ in the solution did not change this parameter significantly. The EIS-EQCM appeared to be a promising tool for an improved characterization and understanding of nonstationary electrochemical interfaces. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jp200755p
  • 2011 • 53 Influence of alkyl chain length and molecular weight on the surface functionalization via adsorption/entrapment with biocidal cationic block copolymers
    Berndt, E. and Behnke, S. and Ulbricht, M.
    European Polymer Journal 47 2379-2390 (2011)
    Polysulfone (PSf) films were functionalized with block copolymers containing poly(n-butyl acrylate) (PBA) as anchor block which is able to firmly tether the biocidal quaternized poly(2-dimethylaminoethyl methacrylate) (PDMAEMAq) to the surface. Block copolymers were synthesized using sequential atom transfer radical polymerization (ATRP) and quaternization with methyl and/or octyl groups rendered the polymers biocidal. Upon reversible swelling of the PSf surface layer in the adsorption/entrapment process, incorporation of the block copolymer is anticipated to be stable; homopolymers, i.e., methyl- or octyl-quaternized PDMAEMAq, were investigated for comparison. The addition of salt to the functionalization solution containing the block copolymer induced a decrease in the critical micelle concentration and lead to higher functionalization efficiency. The impact of intra- or interchain interactions in these aggregates on adsorption and firm entrapment in PSf was determined by measuring contact angle, charge density and zeta potential. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.eurpolymj.2011.09.019
  • 2011 • 52 Concentration-induced wetting transition in water-tetrahydrofuran-isobutane systems
    Boewer, L. and Paulus, M. and Lehmkühler, F. and Tolan, M.
    Journal of Physical Chemistry C 115 18235-18238 (2011)
    The pressure-dependent wetting of isobutane at the aqueous tetrahydrofuran (THF) solution-isobutane interface was studied by means of X-ray reflectivity measurements. Using pure water and mixtures at low THF concentrations, a completely wetting isobutane layer is adsorbed onto the substrate. The pressure-dependent layer thickness can be described by a simple adsorption isotherm. In contrast, the formation of thick layers with low electron density is observed at high THF concentrations. The film growth shows an unpredictable behavior. This finding can be explained by the formation of partially wetting isobutane droplets on the water/THF substrate caused by a decrease of the liquids surface tension with increasing THF concentration. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jp204663w
  • 2011 • 51 In-situ optical spectroscopy and electronic properties of pyrrole sub-monolayers on Ga-rich GaAs(001)
    Bruhn, T. and Ewald, M. and Fimland, B.-O. and Kneissl, M. and Esser, N. and Vogt, P.
    Journal of Nanoparticle Research 13 5847-5853 (2011)
    We report on the characterization of submonolayers of pyrrole adsorbed on Ga-rich GaAs(001) surfaces. The interfaces were characterized by scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) and reflectance anisotropy spectroscopy (RAS) in a spectral range between 1.5 and 8 eV. The adsorption of pyrrole on Ga-rich GaAs(001) modifies the RAS spectrum of the clean GaAs surface significantly at the surface transitions at 2.2 and 3.5 eV indicating a chemisorption of the molecules. By the help of transients at these surface transitions during the adsorption process, we were able to prepare different molecular coverages from a sub-monolayer up to a complete molecular layer. The different coverages of pyrrole were imaged by STM and electronically characterized by STS. The measurements reveal that the adsorbed molecules electronically insulate the surface and indicate the formation of new interface states around-3.5 and +4.2 eV. The RAS measurements in the UV region show new anisotropies in the spectral range of the optical transitions of the adsorbed pyrrole molecules. Our measurements demonstrate the potential of optical and electronic spectroscopy methods for the characterization of atomically thin molecular layers on semiconductor surfaces allowing a direct access to the properties of single adsorbed molecules. © Springer Science+Business Media B.V. 2011.
    view abstractdoi: 10.1007/s11051-011-0340-0
  • 2011 • 50 Synthesis of active carbon-based catalysts by chemical vapor infiltration for nitrogen oxide conversion
    Busch, M. and Bergmann, U. and Sager, U. and Schmidt, W. and Schmidt, F. and Notthoff, C. and Atakan, B. and Winterer, M.
    Journal of Nanoscience and Nanotechnology 11 7956-7961 (2011)
    Direct reduction of nitrogen oxides is still a challenge. Strong efforts have been made in developing noble and transition metal catalysts on microporous support materials such as active carbons or zeolites. However, the required activation energy and low conversion rates still limit its break-through. Furthermore, infiltration of such microporous matrix materials is commonly performed by wet chemistry routes. Deep infiltration and homogeneous precursor distribution are often challenging due to precursor viscosity or electrostatic shielding and may be inhibited by pore clogging. Gas phase infiltration, as an alternative, can resolve viscosity issues and may contribute to homogeneous infiltration of precursors. In the present work new catalysts based on active carbon substrates were synthesized via chemical vapor infiltration. Iron oxide nano clusters were deposited in the microporous matrix material. Detailed investigation of produced catalysts included nitrogen oxide adsorption, X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Catalytic activity was studied in a recycle flow reactor by time-resolved mass spectrometry at a temperature of 423 K. The infiltrated active carbons showed very homogeneous deposition of iron oxide nano clusters in the range of below 12 to 19 nm, depending on the amount of infiltrated precursor. The specific surface area was not excessively reduced, nor was the pore size distribution changed compared to the original substrate. Catalytic nitrogen oxides conversion was detected at temperatures as low as 423 K. Copyright © 2011 American Scientific Publishers. All rights reserved.
    view abstractdoi: 10.1166/jnn.2011.5074
  • 2011 • 49 All-inorganic core-shell silica-titania mesoporous colloidal nanoparticles showing orthogonal functionality
    Cauda, V. and Szeifert, J.M. and Merk, K. and Fattakhova-Rohlfing, D. and Bein, T.
    Journal of Materials Chemistry 21 13817-13824 (2011)
    Colloidal mesoporous silica (CMS) nanoparticles with a thin titania-enriched outer shell showing a spatially resolved functionality were synthesized by a delayed co-condensation approach. The titania-shell can serve as a selective nucleation site for the growth of nanocrystalline anatase clusters. These fully inorganic pure silica-core titania-enriched shell mesoporous nanoparticles show orthogonal functionality, demonstrated through the selective adsorption of a carboxylate-containing ruthenium N3-dye. UV-Vis and fluorescence spectroscopy indicate the strong interaction of the N3-dye with the titania-phase at the outer shell of the CMS nanoparticles. In particular, this interaction and thus the selective functionalization are greatly enhanced when anatase nanocrystallites are nucleated at the titania-enriched shell surface. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c0jm04528d
  • 2011 • 48 Solid-solid interface adsorption of proteins and enzymes in nanophase-separated amphiphilic conetworks
    Dech, S. and Cramer, T. and Ladisch, R. and Bruns, N. and Tiller, J.C.
    Biomacromolecules 12 1594-1601 (2011)
    Amphiphilic polymer conetworks (APCNs) are materials with a very large interface between their hydrophilic and hydrophobic phases due to their nanophase-separated morphologies. Proteins were found to enrich in APCNs by up to 2 orders of magnitude when incubated in aqueous protein solutions, raising the question of the driving force of protein uptake into APCNs. The loading of poly(2-hydroxyethyl acrylate)-linked by-poly(dimethylsiloxane) (PHEA-l-PDMS) with heme proteins (myoglobin, horseradish peroxidase, hemoglobin) and lipases was studied under variation of parameters such as incubation time, pH, concentration of the protein solution, and conetwork composition. Adsorption of enzymes to the uncharged interface is the main reason for protein uptake, resulting in protein loading of up to 23 wt %. Experimental results were supported by computation of electrostatic potential maps of a lipase, indicating that hydrophobic patches are responsible for the adsorption to the interface. The findings underscore the potential of enzyme-loaded APCNs in biocatalysis and as sensors. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/bm1015877
  • 2011 • 47 Reduced protein adsorption by osmolytes
    Evers, F. and Steitz, R. and Tolan, M. and Czeslik, C.
    Langmuir 27 6995-7001 (2011)
    Osmolytes are substances that affect osmosis and are used by cells to adapt to environmental stress. Here, we report a neutron reflectivity study on the influence of some osmolytes on protein adsorption at solid-liquid interfaces. Bovine ribonuclease A (RNase) and bovine insulin were used as model proteins adsorbing at a hydrophilic silica and at a hydrophobic polystyrene surface. From the neutron reflectivity data, the adsorbed protein layers were characterized in terms of layer thickness, protein packing density, and adsorbed protein mass in the absence and presence of urea, trehalose, sucrose, and glycerol. All data point to the clear effect of these nonionic cosolvents on the degree of protein adsorption. For example, 1 M sucrose leads to a reduction of the adsorbed amount of RNase by 39% on a silica surface and by 71% on a polystyrene surface. Trehalose was found to exhibit activity similar to that of sucrose. The changes in adsorbed protein mass can be attributed to a decreased packing density of the proteins in the adsorbed layers. Moreover, we investigated insulin adsorption at a hydrophobic surface in the absence and presence of glycerol. The degree of insulin adsorption is decreased by even 80% in the presence of 4 M of glycerol. The results of this study demonstrate that nonionic cosolvents can be used to tune and control nonspecific protein adsorption at aqueous-solid interfaces, which might be relevant for biomedical applications. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/la2010908
  • 2011 • 46 Real-space imaging of inelastic Friedel-like surface oscillations emerging from molecular adsorbates
    Gawronski, H. and Fransson, J. and Morgenstern, K.
    Nano Letters 11 2720-2724 (2011)
    We report real space imaging measurements of inelastic Friedel oscillations. The inelastic electron tunneling spectroscopy, using scanning tunneling microscopy, around dimers of dichlorobenze adsorbates on Au(111) surface display clear spatial modulations that we attribute to inelastic scattering at the molecular sites caused by molecular vibrations. Due to local interactions between the adsorbate and the surface states, the molecular vibrations generate a redistribution of the charge density at energies in a narrow range around the inelastic mode. Our experimental findings are supported by theoretical arguments. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/nl201076g
  • 2011 • 45 The influence of the potassium promoter on the kinetics and thermodynamics of CO adsorption on a bulk iron catalyst applied in Fischer-Tropsch synthesis: A quantitative adsorption calorimetry, temperature-programmed desorption, and surface hydrogenation study
    Graf, B. and Muhler, M.
    Physical Chemistry Chemical Physics 13 3701-3710 (2011)
    The adsorption of carbon monoxide on an either unpromoted or potassium-promoted bulk iron catalyst was investigated at 303 K and 613 K by means of pulse chemisorption, adsorption calorimetry, temperature-programmed desorption and temperature-programmed surface reaction in hydrogen. CO was found to adsorb mainly molecularly in the absence of H 2 at 303 K, whereas the presence of H 2 induced CO dissociation at higher temperatures leading to the formation of CH 4 and H 2O. The hydrogenation of atomic oxygen chemisorbed on metallic iron was found to occur faster than the hydrogenation of atomically adsorbed carbon. At 613 K CO adsorption occurred only dissociatively followed by recombinative CO 2 formation according to C ads + 2O ads → CO 2(g). The presence of the potassium promoter on the catalyst surface led to an increasing strength of the Fe-C bond both at 303 K and 613 K: the initial differential heat of molecular CO adsorption on the pure iron catalyst at 303 K amounted to 102 kJ mol -1, whereas it increased to 110 kJ mol -1 on the potassium-promoted sample, and the initial differential heat of dissociative CO adsorption on the unpromoted iron catalyst at 613 K amounted to 165 kJ mol -1, which increased to 225 kJ mol -1 in the presence of potassium. The calorimetric CO adsorption experiments also reveal a change of the energetic distribution of the CO adsorption sites present on the catalyst surface induced by the potassium promoter, which was found to block a fraction of the CO adsorption sites. © the Owner Societies 2011.
    view abstractdoi: 10.1039/c0cp01875a
  • 2011 • 44 Hydrothermally highly stable acidic mesoporous aluminosilicate spheres with radial channels
    Gu, X. and Jiang, T. and Tao, H. and Zhou, S. and Liu, X. and Ren, J. and Wang, Y. and Lu, G. and Schmidt, W.
    Journal of Materials Chemistry 21 880-886 (2011)
    Hydrothermally highly stable mesoporous aluminosilicate spheres with radial channels were synthesized in the CTAB-NaF-TPAOH system through a one-step procedure at high aging temperature. The characterization by transmission electron microscopy (TEM), X-ray diffraction (XRD), nitrogen adsorption/desorption analysis, 27Al MAS solid state NMR spectroscopy, pyridine adsorption FT-IR combined with the typical hydrothermal treatments showed that this kind of material exhibited large surface area, specific pore arrangement, strong acidity and high hydrothermal stability. Detailed studies suggest that F- ions direct the perpendicular arrangement of aluminosilicate clusters during the hydrothermal treatment at 160°C, while TPA+ stabilized the structure. Both F- and TPA+ ions are considered to improve the acidity and hydrothermal stability of this material through coordination of framework atoms, thus, enhancing the condensation of Si-O-Si bonds in the amorphous pore walls. Due to the accessible radial pore arrangement and high acidity, the catalytic activity for Friedel-Crafts alkylation of toluene with benzyl alcohol was excellent with 100% conversion of benzyl alcohol.
    view abstractdoi: 10.1039/c0jm01973a
  • 2011 • 43 Co3O4 - SiO2 Nanocomposite: A very active catalyst for co oxidation with unusual catalytic behavior
    Jia, C.-J. and Schwickardi, M. and Weidenthaler, C. and Schmidt, W. and Korhonen, S. and Weckhuysen, B.M. and Schüth, F.
    Journal of the American Chemical Society 133 11279-11288 (2011)
    A high surface area Co3O4 - SiO2 nanocomposite catalyst has been prepared by use of activated carbon as template. The Co3O4 - SiO2 composite, the surface of which is rich in silica and Co(II) species compared with normal Co 3O4, exhibited very high activity for CO oxidation even at a temperature as low as '76 °C. A rather unusual temperature-dependent activity curve, with the lowest conversion at about 80 °C, was observed with a normal feed gas (H2O content 3 ppm). The U-shape of the activity curve indicates a negative apparent activation energy over a certain temperature range, which has rarely been observed for the heterogeneously catalyzed oxidation of CO. Careful investigation of the catalytic behavior of Co 3O4 - SiO2 catalyst led to the conclusion that adsorption of H2O molecules on the surface of the catalyst caused the unusual behavior. This conclusion was supported by in situ diffuse reflectance Fourier transform infrared (DRIFT) spectroscopic experiments under both normal and dry conditions. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja2028926
  • 2011 • 42 Magnitude and time response of electronic and topographical changes during hydrogen sensing in size selected palladium nanoparticles
    Khanuja, M. and Shrestha, S. and Mehta, B.R. and Kala, S. and Kruis, F.E.
    Journal of Applied Physics 110 (2011)
    In this study, size dependence of electronic and topographical effects during Pd-H interaction has been investigated by studying H sensing in thin films of size selected and monosized nanoparticles having 15, 20, and 25 nm diameter. By separating the contributions of electronic changes due to H adsorption and topographical changes due to lattice expansion to hydrogen sensing, it has been shown that the magnitude and response time of these changes are sensitive functions of nanoparticle size and measurement temperature. The temperature dependence of saturated resistance corresponding to these changes provides important information about the nature of electronic and topographical changes. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3603053
  • 2011 • 41 The stretching vibration of hydrogen adsorbed on epitaxial graphene studied by sum-frequency generation spectroscopy
    Kim, H. and Balgar, T. and Hasselbrink, E.
    Chemical Physics Letters 508 1-5 (2011)
    IR-vis sum-frequency generation spectroscopy is used to study the stretching vibration of hydrogen chemically bound to a graphene sheet prepared on an Ir (1 1 1) crystal surface. We observe two distinct resonances at 2563 and 2716 cm-1. Similarly, lines at 1881 and 2027 cm-1 are found after D atom adsorption. The assignment to C-H stretching vibrations is discussed in view of the propensity for dimer formation reported for hydrogen adsorbed on graphene. The lines are assigned to hydrogen bound in either ortho or para configuration to the hexagonal mesh of carbon atoms. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cplett.2011.04.025
  • 2011 • 40 Alginate/poly-l-lysine capsules: Mechanical properties and drug release characteristics
    Leick, S. and Kemper, A. and Rehage, H.
    Soft Matter 7 6684-6694 (2011)
    In this paper we studied the mechanical stability and the release kinetics of different types of liquid-filled calcium alginate/poly-l-lysine capsules. The aqueous cores of these particles were filled with anthocyanins which have antioxidant abilities and may, as additives in foods, provide several benefits of health. By means of spinning capsule experiments it was possible to measure the deformation of the gel particles at various centrifugal forces. These investigations provided insight into the elastic properties of the capsule membranes. In a series of experiments we measured the capsule deformation as a function of the poly-l-lysine concentration and the adsorption time. From these data we calculated the surface Young moduli. In addition, the use of a pH-dependent UV/VIS-absorption spectroscopy method gained access to the diffusive drug-release performance of the encapsulated anthocyanins. From these kinetic measurements we could evaluate the effective diffusion constants of the encapsulated compounds. The performed experiments showed that the mechanical properties of liquid-filled alginate capsules could be changed and adjusted selectively by the addition of poly-l-lysine. The drug release properties, however, did not change significantly for different compositions of the multi-component capsules. In addition, it could be shown that a high amount of anthocyanin molecules was immobilized in the capsules. This phenomenon could be explained by adsorption or polymerization processes of the colored ingredients. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c1sm05676j
  • 2011 • 39 Softening of PMMA brushes upon collapse/swelling transition. A combined neutron reflectivity and nanomechanical cantilever sensor study
    Lenz, S. and Rühm, A. and Major, J. and Berger, R. and Gutmann, J.S.
    Macromolecules 44 360-367 (2011)
    In this work, we establish a direct correlation between chain mechanics and structural properties of polymer brushes upon swelling. We present experimental results on poly(methyl methacrylate) (PMMA) brushes prepared via surface initiated atomic transfer radical polymerization. Neutron reflectivity studies gave insight into the brush thickness and volume fraction profiles of the brush, gradually swollen with solvent mixtures. Comparison of our experiments with scaling theory yielded specific polymer - solvent interaction parameters and gave insight into the desorption and adsorption behavior of bad and good solvents, respectively. Insight into the brush's chain mechanics was obtained from surface stress investigations using the nanomechanical cantilever sensor bending technique. It was shown that polymer brush swelling leads to a decrease in surface stress due to chain disentanglements and the related softening of the polymer brush under θ-solvent conditions. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ma1021715
  • 2011 • 38 High-pressure CO adsorption on Cu-based catalysts: Zn-induced formation of strongly bound CO monitored by ATR-IR spectroscopy
    Liu, Z. and Rittermeier, A. and Becker, M. and Kähler, K. and Löffler, E. and Muhler, M.
    Langmuir 27 4728-4733 (2011)
    CO adsorption at 1 MPa on Cu-Zn stearate colloids and supported Cu catalysts was studied in situ by attenuated total reflection infrared (ATR-IR) spectroscopy. Subsequent to thorough reduction by H2, the IR band at 2110-2070 cm-1 due to linearly adsorbed CO on clean metallic Cu was always observed initially on all Cu catalysts. During the exposure of Zn-containing samples to CO at high pressure, a new IR band at ca. 1975 cm -1 appeared in addition and increased in intensity even at room temperature. The detailed analysis of the IR spectra showed that the new IR band at ca. 1975 cm-1 was not related to coadsorbed carbonate/formate- like species, but to the content of Zn in the samples. This IR band was found to be more stable than that at 2110-2070 cm-1 during purging with inert gas. It disappeared quickly in synthetic air, pointing to a strongly reduced state of the Zn-containing Cu catalysts achieved during high-pressure CO exposure. It is suggested that CO can reduce ZnO to Zn in the presence of Cu, resulting in the formation of a CuZnx surface alloy. As the CO species with the characteristic IR band at ca. 1975 cm-1 binds more strongly to this CuZnx alloy than the linearly adsorbed CO to pure Cu, it is suggested to be adsorbed on a bridge site. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/la2000766
  • 2011 • 37 Stepwise deposition of metal organic frameworks on flexible synthetic polymer surfaces
    Meilikhov, M. and Yusenko, K. and Schollmeyer, E. and Mayer, C. and Buschmann, H.-J. and Fischer, R.A.
    Dalton Transactions 40 4838-4841 (2011)
    Thin films of [Cu3(btc)2]n (btc = 1,3,5-benzenetricarboxylate) metal organic framework were deposited in a stepwise manner on surfaces of flexible organic polymers. The thickness of films can be precisely controlled. The deposition of the first cycles was monitored by UV-vis spectroscopy. The porosity was proven by the adsorption of pyrazine, which was monitored by FT-IR and thermogravimetric analysis. The deposition of MOF thin films on flexible polymer surfaces might be a new path for the fabrication of functional materials for different applications, such as protection layers for working clothes and gas separation materials in the textile industry. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c0dt01820a
  • 2011 • 36 Ultrafast dynamics at the Na/D2O/Cu(111) interface: Electron solvation in Ice layers and Na+-mediated surface solvation
    Meyer, M. and Bertin, M. and Bovensiepen, U. and Wegkamp, D. and Krenz, M. and Wolf, M.
    Journal of Physical Chemistry C 115 204-209 (2011)
    We have studied the influence of sodium ions bound near the ice/vacuum interface on the electron solvation dynamics in amorphous D2O ice layers by means of femtosecond time-resolved two-photon photoelectron spectroscopy. Adsorption of submonolayer coverages of sodium on top of multilayers of amorphous ice leads to the formation of Na+ ions and to pronounced changes in the observed ultrafast dynamics compared to pure amorphous ice. We identify a Na+-induced species of excess electrons which exhibits a much longer lifetime compared to excess electrons in pure D2O ice and approximate the decay of the Na-induced contribution by two decay times τ2 = 880 fs and τ3 = 9.6 ps. In addition, a faster energetic stabilization of the excited electrons with a rate of σ = 0.73 eV/ps is observed. The population of these electrons depends nonlinearly on the sodium coverage. We attribute the Na-induced contribution to a transient electron/ion/water complex which is located at the ice/vacuum interface. This interpretation is corroborated by coverage-dependent measurements and by overlayer experiments. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp107253g
  • 2011 • 35 Simultaneous correlation of hydrophobic interactions in HIC and protein solubility in aqueous salt solutions and mixed solvents
    Mollerup, J.M. and Breil, M.P. and Vogelpohl, C. and Sadowski, G.
    Fluid Phase Equilibria 301 163-170 (2011)
    The chromatographic retention in hydrophobic and reversed phase chromatography and the solubility of proteins display some common features. The chromatographic retention, as well as the solubility, is modulated by the thermodynamic properties of the solute in the fluid phase. The retention measurements at linear conditions provide information of the solution properties of the protein at infinite dilution, and the solubility measurements produce the supplementary information about the solution properties at the saturation limit. This provides a useful approach to simultaneous correlation of the chromatographic retention and the solubility.The experimental data, used for the correlation, comprise retention measurements of lysozyme on different HIC adsorbents using an aqueous ammonium sulphate eluant, an aqueous ammonium sulphate eluant with an admixture of ethanol, as well as published solubility data.The chromatographic retention data and the corresponding solubility data have been correlated using a chemical potential model derived from Kirkwood's theory of solutions of charged macro-ions and zwitterions in electrolyte solutions. The model correlated the chromatographic retention factor and the solubility data within the precision of the measurements. The model was applied in a pH range from 4 to 11. It was demonstrated experimentally, as well as theoretically, that an admixture of ethanol to the aqueous eluant changes the thermodynamic retention factor on various adsorbents identically when compared to the thermodynamic retention factor in an ethanol free eluant. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.fluid.2010.11.028
  • 2011 • 34 The role of the synthesis method in the structure formation of cobalt aluminate
    Onfroy, T. and Li, W.-C. and Schüth, F. and Knözinger, H.
    Topics in Catalysis 54 390-397 (2011)
    Cobalt aluminate (spinel) was prepared via two synthesis routes firstly a co-precipitation method and secondly a nanocasting method. The surface chemistry of these materials was characterized by infrared-spectroscopy of the surface hydroxyl groups and of coordinatively unsaturated (cus) cations (Al and Co) by carbon monoxide CO at low temperature. The goal was to investigate whether or not the preparation of the spinel phase had an effect on this structural characteristics, namely on the inversion degree. The hydroxyl (deuteroxyl) spectra were characterized by six types of O-H stretching bands. While the O-H stretching frequencies were identical for both materials, the relative intensities of the bands were clearly different indicating a distinct distribution of the different types of hydroxyl groups which most likely originates from a distinct distribution of the cations in the two differently synthesized materials. The preparation method obviously yields spinel structures having variable degrees of inversion. Coordinatively unsaturated cus cations are exposed on the surface during the removal of O-H groups by thermal dehydroxylation. Those Lewis acid centers show an acid strength distribution which is again an indication for formation of two distinct degrees of inversion. © 2011 Springer Science+Business Media, LLC.
    view abstractdoi: 10.1007/s11244-011-9669-y
  • 2011 • 33 Adsorptive water removal from organic solvents in the ppm-region
    Pahl, C. and Pasel, C. and Luckas, M. and Bathen, D.
    Chemie-Ingenieur-Technik 83 177-182 (2011)
    Highly pure organic solvents gain more and more importance in optical, electronical, pharmaceutical and chemical industry. In this context the removal of water in the lower ppm- and ppb-range is a major challenge since water is always present, e. g., air humidity, and leads to catalyst poisoning and undesired side reactions. One technique to remove water is by adsorption on zeolites, silica gels or aluminas. Despite single technical solutions already existing, systematic measurements of influencing parameters are still missing. The research project adresses the adsorption of water from different organic solvents. In the following experiments with linear primary alcohols and acetic acid esters are discussed as these solvents are often used in industry in low water content qualities. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201000168
  • 2011 • 32 Structure, chemical composition, and reactivity correlations during the in situ oxidation of 2-propanol
    Paredis, K. and Ono, L.K. and Mostafa, S. and Li, L. and Zhang, Z. and Yang, J.C. and Barrio, L. and Frenkel, A.I. and Cuenya, B.R.
    Journal of the American Chemical Society 133 6728-6735 (2011)
    Unraveling the complex interaction between catalysts and reactants under operando conditions is a key step toward gaining fundamental insight in catalysis. We report the evolution of the structure and chemical composition of size-selected micellar Pt nanoparticles (∼1 nm) supported on nanocrystalline γ-Al2O3 during the catalytic oxidation of 2-propanol using X-ray absorption fine-structure spectroscopy. Platinum oxides were found to be the active species for the partial oxidation of 2-propanol (< 140 °C), while the complete oxidation (&gt;140 °C) is initially catalyzed by oxygen-covered metallic Pt nanoparticles, which were found to regrow a thin surface oxide layer above 200 °C. The intermediate reaction regime, where the partial and complete oxidation pathways coexist, is characterized by the decomposition of the Pt oxide species due to the production of reducing intermediates and the blocking of O2 adsorption sites on the nanoparticle surface. The high catalytic activity and low onset reaction temperature displayed by our small Pt particles for the oxidation of 2-propanol is attributed to the large amount of edge and corner sites available, which facilitate the formation of reactive surface oxides. Our findings highlight the decisive role of the nanoparticle structure and chemical state in oxidation catalytic reactions. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ja200178f
  • 2011 • 31 SrTiO3 nanotubes with negative strain energy predicted from first principles
    Piskunov, S. and Spohr, E.
    Journal of Physical Chemistry Letters 2 2566-2570 (2011)
    On the basis of hybrid density functional theory calculations, we predict that the most energetically favorable single-walled SrTiO3 nanotubes with negative strain energy can be folded from SrTiO3 (110) nanosheets of rectangular morphology. Further formation of multiwalled tubular nanostructure with interwall distance of ∼0.46 nm yields an additional gain in energy of 0.013 eV per formula unit. (The formation energy of the most stable nanotube is 1.36 eV/SrTiO3.) Because of increase in the Ti-O bond covalency in the outer shells, SrTiO3 nanotubes can demonstrate an enhancement of their adsorption properties. Quantum confinement leads to a widening of the energy band gap of single-walled SrTiO3 nanotubes (∼6.1 eV) relative to the bulk (∼3.6 eV), which makes them attractive for further band gap engineering. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/jz201050e
  • 2011 • 30 Chemical composition of surface-functionalized gold nanoparticles
    Rostek, A. and Mahl, D. and Epple, M.
    Journal of Nanoparticle Research 13 4809-4814 (2011)
    The composition of surface-functionalized gold nanoparticles (diameter of the metallic core: 17-20 nm) was determined by elemental analysis (C, H, N, S, Au, Na) after preparation of a larger batch. Gold nanoparticles were prepared and functionalized with citrate according to the classical Turkevich method. The citrate-functionalized nanoparticles contained about 3.1 wt% of organic material (135 ng cm-2 or 3.1 molecules nm-2). A partial exchange of citrate was accomplished by tris(sodium-m-sulfonato-phenyl)phosphine (TPPTS) which led to 2.1 wt% of citrate (90 ng cm-2 or 2.1 molecules nm-2) and 1.4 wt% TPPTS (61 ng cm-2 or 0.6 molecules nm-2). The citrate coating was quantitatively exchanged by poly(N-vinyl pyrrolidone) (PVP) after immersion in solutions with concentrations of 33, 66 and 128 mg L-1, respectively, leading to contents of 4 to 6 wt% of PVP (171-271 ng cm-2 or 9-15 PVP monomer units nm -2). © 2011 Springer Science+Business Media B.V.
    view abstractdoi: 10.1007/s11051-011-0456-2
  • 2011 • 29 Janus cylinders at liquid-liquid interfaces
    Ruhland, T.M. and Gröschel, A.H. and Walther, A. and Müller, A.H.E.
    Langmuir 27 9807-9814 (2011)
    We describe the first study on the self-assembly behavior of Janus cylinders at liquid/liquid interfaces. The Janus cylinders are characterized by a phase separation along the major axis into two hemicylinders of different wettability. The pendant drop technique and microscopic imaging were used to characterize the adsorption behavior and self-assembly of Janus cylinders at perfluorinated oil/dioxane and perfluorinated oil/dimethyl sulfoxide interfaces. According to the evolution of the interfacial tension and a series of TEM images taken during the cylinder adsorption, we will specify the characteristics of early to late stages of the Janus cylinder adsorption at a liquid-liquid interface and discuss the effect of Janus cylinder length and their concentration. We also establish that the broken symmetry of the corona leads to significantly higher interfacial activity as compared to homogeneous core-shell cylinders. The adsorption is characterized by three different adsorption stages: first, free diffusion to the interface, followed by continuous adsorption of cylinders including ordering and domain formation and, finally, additional packing with a rearrangement of domains and formation of a loose multilayer system. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/la201863x
  • 2011 • 28 Investigating the gas phase emitter effect of caesium and cerium in ceramic metal halide lamps in dependence on the operating frequency
    Ruhrmann, C. and Westermeier, M. and Bergner, A. and Luijks, G.M.J.F. and Awakowicz, P. and Mentel, J.
    Journal of Physics D: Applied Physics 44 (2011)
    The work function and with it the temperature of tungsten electrodes in HID lamps can be lowered and the lifetime of lamps increased by the gas phase emitter effect. A determination of the emitter effect of Cs and Ce is performed by phase resolved measurements of the electrode tip temperature T tip(φ), plasma temperature Tpl(φ) and particle densities N(φ) by means of pyrometric, optical emission and broadband absorption spectroscopy in dependence on the operating frequency. The investigated HID lamps are ceramic metal halide lamps with transparent discharge vessels made of YAG, filled with a buffer gas consisting of Ar, Kr and predominantly Hg and seeded with CsI or CeI3. In the YAG lamp seeded with CsI and CeI3 as well as in a YAG lamp seeded with DyI 3 (corresponding results can be found in a preceding paper) a gas phase emitter effect is observed in the cathodic phase due to a Cs, Ce or Dy ion current. In the YAG lamp seeded with CsI the phase averaged coverage of the electrode surface with emitter atoms decreases and the electrode temperature rises with increasing frequency, whereas the emitter effect of Ce and Dy is extended to the anodic phase, which leads to a decreased average temperature Ttip(φ) with increasing frequency. This different behaviour of the averaged values of Ttip(φ) for increasing frequency is caused by the differing adsorption energies Ea of the respective emitter materials. In spite of the influence of Ea on the coverage of the electrode with emitter atoms, the cathodic gas phase emitter effect produces in the YAG lamps seeded with CsI, CeI3 and DyI3 a general reduction in the electrode tip temperature Ttip(φ) in comparison with a YAG lamp with Hg filling only. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/44/35/355202
  • 2011 • 27 Nanostructured Ti-catalyzed MgH2 for hydrogen storage
    Shao, H. and Felderhoff, M. and Schüth, F. and Weidenthaler, C.
    Nanotechnology 22 (2011)
    Nanocrystalline Ti-catalyzed MgH2 can be prepared by a homogeneously catalyzed synthesis method. Comprehensive characterization of this sample and measurements of hydrogen storage properties are discussed and compared to a commercial MgH2 sample. The catalyzed MgH2 nanocrystalline sample consists of two MgH2 phases-a tetrahedral β-MgH2 phase and an orthorhombic high-pressure modification γ-MgH2. Transmission electron microscopy was used for the observation of the morphology of the samples and to confirm the nanostructure. N2 adsorption measurement shows a BET surface area of 108m 2g-1 of the nanostructured material. This sample exhibits a hydrogen desorption temperature more than 130 °C lower compared to commercial MgH2. After desorption, the catalyzed nanocrystalline sample absorbs hydrogen 40 times faster than commercial MgH2 at 300 °C. Both the Ti catalyst and the nanocrystalline structure with correspondingly high surface area are thought to play important roles in the improvement of hydrogen storage properties. The desorption enthalpy and entropy values of the catalyzed MgH2 nanocrystalline sample are 77.7kJmol-1H2 and 138.3JK-1mol -1H2, respectively. Thermodynamic properties do not change with the nanostructure. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/22/23/235401
  • 2011 • 26 Infrared spectroscopic ellipsometry (IRSE) and X-ray photoelectron spectroscopy (XPS) monitoring the preparation of maleimide-functionalized surfaces: From Au towards Si (111)
    Sun, G. and Hovestädt, M. and Zhang, X. and Hinrichs, K. and Rosu, D.M. and Lauermann, I. and Zielke, C. and Vollmer, A. and Löchel, H. and Ay, B. and Holzhütter, H.-G. and Schade, U. and Esser, N. and Volkmer, R. and Rappich, J.
    Surface and Interface Analysis 43 1203-1210 (2011)
    The IR ellipsometric technique was used to identify the surface species and to control the preparation of maleimide-terminated surfaces. Because of higher s/n ratios for metallic substrates, the protocol was initially developed on Au surfaces, was later successfully transferred to technologically more relevant Si (111) substrates. The functionalized surfaces were achieved by electrochemical deposition of diazonium linker films and following chemical adsorption steps. Complementary XPS was also employed to detect the surface species in the process of preparation. The immobilization of different functional molecules was proven by interpreting the specific vibrational bands in IR spectra and additionally confirmed by XPS experiments. The surface homogeneity was investigated by FT-IR synchrotron mapping ellipsometry. This work shows that the proposed protocol is an effective pathway to achieve the desired functionalized surfaces. Copyright © 2010 John Wiley & Sons, Ltd.
    view abstractdoi: 10.1002/sia.3699
  • 2011 • 25 Amphiphilic gold nanoparticles: Synthesis, characterization and adsorption to PEGylated polymer surfaces
    Tarnawski, R. and Ulbricht, M.
    Colloids and Surfaces A: Physicochemical and Engineering Aspects 374 13-21 (2011)
    The direct synthesis of water-soluble gold nanoparticles with a mixed shell of two different thiols, 1-mercaptoundec-11-yl-hexa(ethylene glycol) (EG6) and dodecanethiol (C12), and their characterization are reported. Data from IR spectroscopy and contact angle (CA) measurements as well as the solubility of the nanoparticles in water support that the composition of the shell is in the range of the thiol ratio used for synthesis (EG6:C12 = 72:28). Results of transmission electron microscopy and atomic force microscopy (AFM) for deposited particles as well as the UV-vis spectrum in solution are in line with a size of ≤10. nm. Self-assembled monolayers (SAMs) as model surfaces were prepared from mixtures of EG6 and C12 on planar gold films. Polystyrene (PSt) spin-coated films on silicon wafers and on gold-coated surface plasmon resonance (SPR) sensor disks were used as substrates for surface functionalization via adsorption/self-assembly of a polystyrene poly(ethylene glycol) diblock copolymer (PSt- b-PEG) from aqueous solutions. CA and AFM results revealed pronounced differences of the hydrophilicity/hydrophobicity and topography of the surface as a function of PSt- b-PEG concentration used for the modification. The adsorption of myoglobin and the novel gold nanoparticles to the PSt- b-PEGylated surfaces was analyzed by SPR. A control of adsorbed amounts by the degree of surface PEGylation, i.e. a reduction by up to 55% for the highest degree of modification, could be confirmed for both kinds of colloids. Adsorption of the novel gold nanoparticles to the mixed SAM surfaces as analyzed by SPR showed an even stronger dependency of surface composition. All experiments demonstrate that amphiphilic, water-soluble gold-based nanoparticles can be used as model colloids for the investigation of interactions with polymer surfaces of varied structure and architecture, and that they could be further developed for analytical or biological applications. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.colsurfa.2010.10.027
  • 2011 • 24 Visualization and functions of surface defects on carbon nanotubes created by catalytic etching
    Xia, W. and Yin, X. and Kundu, S. and Sánchez, M. and Birkner, A. and Wöll, C. and Muhler, M.
    Carbon 49 299-305 (2011)
    Surface defects were created on carbon nanotubes (CNTs) by catalytic steam gasification or catalytic etching with iron as catalysts. The structure and morphology of the etched CNTs were studied by transmission electron microscopy (TEM) and scanning tunneling microscopy (STM). The electronic structure of the etched CNTs was investigated by ultraviolet photoelectron spectroscopy (UPS). The etched CNTs were treated by nitric acid to obtain oxygen-containing functional groups. The amount and the thermal stability of these groups were studied by temperature-resolved X-ray photoelectron spectroscopy (XPS). Temperature-programmed desorption with ammonia as a probe molecule (NH 3-TPD) was employed to investigate the interaction of the surface defects with foreign molecules in gas phase. TEM and STM studies disclosed the presence of surface defects especially edge planes on the etched CNTs. Etching of CNTs led to a less pronounced p-π band than the as-is CNTs, as evidenced by UPS studies. The XPS and NH 3-TPD studies demonstrated that the defects on the CNTs enhanced the reactivity of the exposed surfaces allowing obtaining a higher degree of oxygen functionalization and more active adsorption sites. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.carbon.2010.09.025
  • 2011 • 23 Composites of functional polymeric hydrogels and porous membranes
    Yang, Q. and Adrus, N. and Tomicki, F. and Ulbricht, M.
    Journal of Materials Chemistry 21 2783-2811 (2011)
    Polymeric hydrogels are a most interesting class of "soft matter" with several established and many more possible applications as functional materials. In this review we will focus on the combination of polymeric hydrogels and porous membranes which leads to composites with promising functionality for, e.g., mass separations, sensing and analytics, (bio)catalysis, biomedical engineering and micro-system technologies. The combination of a rigid porous membrane with a soft functional hydrogel by a suited preparation technique enables that the functionality of the hydrogel can be applied in a unique way. The most important preparation strategies for hydrogel composite membranes, i.e., pore-filling, various surface-grafting methods and combinations thereof, will be discussed. The structural diversity of the hydrogels is based on the use of a wide range of synthetic monomers, but biopolymers or their derivatives can also be applied. The interplay of the membrane pore structure, the structure of the hydrogel and the distribution of the hydrogel in the pore space can lead to different types of composite membranes with completely different potential applications. The focus will be on promising examples for the various types of functional composite membranes, i.e., macroporous membrane adsorbers, anti-fouling filtration membranes, hydrogel-based ultrafiltration membranes, other separation membranes with pore-filling hydrogel as selective material, stimuli-responsive membranes and porous membrane valves and gates, as well as biocompatible or bioactive membranes. © The Royal Society of Chemistry 2011.
    view abstractdoi: 10.1039/c0jm02234a
  • 2010 • 22 γ-Fe2O3 nanoparticle adsorption at an OTS Langmuir monolayer
    Degen, P. and Paulus, M. and Leick, S. and Tolan, M. and Rehage, H.
    Colloid and Polymer Science 288 643-651 (2010)
    The assembling of magnetic nanoparticles in ordered structures as well as the preparation of very thin magnetic switchable polymer membranes is an important aim in many technical fields. We studied the influence of γ-Fe2O3 nanoparticles on the polymerization process and on the properties of the poly(organosiloxane)/nanoparticle-composite layer by surface rheological measurements, surface pressure/area (π/A) isotherm measurements, and Brewster angle microscopy. The adsorption process dynamics were studied by X-ray reflectivity and surface potential measurements. The results confirm the presence of attractive electrostatic interactions between the partial negatively charged monolayer and the positively charged nanoparticles. For further investigations, we prepared Langmuir-Blodgett layers of these polymer-nanoparticle composite and investigated them by atomic force microscopy and UV-Vis spectroscopy. We found that the concentration of nanoparticles was very low and the particles were mainly arranged below the polymer layer. © 2010 Springer-Verlag.
    view abstractdoi: 10.1007/s00396-010-2191-0
  • 2010 • 21 Probing adsorption and aggregation of insulin at a poly(acrylic acid) brush
    Evers, F. and Reichhart, C. and Steitz, R. and Tolan, M. and Czeslik, C.
    Physical Chemistry Chemical Physics 12 4375-4382 (2010)
    A planar poly(acrylic acid) (PAA) brush provides an unusual substrate for the unspecific immobilization of proteins on material surfaces. At neutral pH-values, proteins adsorb at a PAA brush when the ionic strength of the protein solution is low. In contrast, raising the ionic strength to a few 100 mM transforms a PAA brush into a rather protein-resistant surface coating. Moreover, a PAA brush represents a mild environment for adsorbed proteins which preserves their secondary structure and biological activity. In this study, we focus on the biocompatibility of a PAA brush with an insulin solution. Insulin can form amyloid fibrils, which may also be triggered by interfaces. Using neutron reflectometry and attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, the effects of pD value, ionic strength, and glycerol concentration on the density profile and the secondary structure of adsorbed insulin molecules at a PAA brush have been studied. At pD 7, insulin adsorbs at a PAA brush despite its negative net charge. As has been found for other proteins in earlier studies, increasing the ionic strength of the insulin solution to 500 mM decreases the amount of adsorbed insulin drastically. In contrast, at pD 2, addition of salt to the insulin solution induces a thick insulin adsorbate at a PAA brush suggesting both protein-brush and protein-protein interactions, i.e., insulin adsorption and aggregation to be effective. However, in the presence of 2 M glycerol, insulin adsorption is largely suppressed. Furthermore, no major alterations of the secondary structure of adsorbed insulin can be detected by ATR-FTIR spectroscopy under all conditions studied. Hence, the performed experiments demonstrate that a PAA brush does not promote the formation of insulin amyloid structures, which represents a fundamentally new aspect of the biocompatibility of this material surface coating. © 2010 the Owner Societies.
    view abstractdoi: 10.1039/b925134k
  • 2010 • 20 Adsorptive removal of alkoxyphenols from ketones and esters with activated carbon - Experiments and modelling
    Gräf, T. and Pasel, C. and Bathen, D.
    Chemie-Ingenieur-Technik 82 1763-1769 (2010)
    Ultra pure chemicals are most notably used in electronics, optics, pharmaceutics and analytics. For the production of ultra pure liquids adsorptive removal of trace compounds and moisture is the preferred treatment. Adsorption experiments with the model system activated carbon/alkoxyphenol in ketones and esters as solvent are presented. Points of interest are the investigation of adsorption mechanisms as well as the measurement and modelling of adsorption isotherms. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cite.201000086
  • 2010 • 19 The effects of (macro)molecular structure on hydrophilic surface modification of polypropylene membranes via entrapment
    Guo, H. and Ulbricht, M.
    Journal of Colloid and Interface Science 350 99-109 (2010)
    Entrapment of a variety of ethyleneoxide-containing substances from nonpolar solutions into polypropylene (PP) microfiltration membrane surface for hydrophilic modification was studied. The results from gravimetric weight gain, surface characterization by contact angle measurements and ATR-IR spectroscopy, water flux measurements and protein adsorption revealed that poly(ethylene glycol)s (PEGs) were ineffective, while many nonionic amphiphilic substances, especially some tri-block copolymers of poly(ethyleneoxide) (PEO) and poly(propylene oxide) (PPO) were very effective for PP surface modification. The relationship between modifier structure and architecture and entrapment behavior was investigated by studying the micellization of the amphiphilic modifiers in nonpolar solutions via pyrene-probe fluorescence and 1H NMR spectroscopy. We observed that the balanced structure of nonionic tri-block (macro)molecules tended to promote the formation of reverse micelles. For the most efficient polymeric modifiers, the lowest reverse critical micelle concentration (RCMC) had been observed. We conclude that a block copolymer structure and architecture promoting the self-association in the nonpolar solvent is the basis for a high modification efficiency, and that reverse micelles are involved in the entrapment modification performed at concentrations above RCMC. A different mechanism has been deduced for amphiphilic modifiers with low molar mass. This work provides more comprehensive insights in surface entrapment as a easy to perform physical surface modification method for polymeric materials. © 2010 Elsevier Inc.
    view abstractdoi: 10.1016/j.jcis.2010.06.032
  • 2010 • 18 Surface modification of polypropylene microfiltration membrane via entrapment of an amphiphilic alkyl oligoethyleneglycolether
    Guo, H. and Ulbricht, M.
    Journal of Membrane Science 349 312-320 (2010)
    For surface hydrophilic and antifouling modification of polypropylene (PP) microfiltration membrane, the novel method for entrapment of the amphiphilic modifier octaethyleneglycol monooctadecylether (C18E8) was investigated in detail. The effects of the modification conditions on PP membrane and polymer structure were characterized by gas flow/pore dewetting, nitrogen adsorption/BET analysis, scanning electron microscopy and X-ray diffraction; surface properties were evaluated by ATR-FTIR spectroscopy and static water contact angle; filtration performance as well as antifouling property were investigated by water flux measurement, trans-membrane zeta potential, static and dynamic protein adsorption experiments. Furthermore, a stability study of the modified membrane was performed to offer a comprehensive understanding of this physical entrapment strategy. It can be concluded that both outer surface and inner pore walls of PP membrane were covered with oligoethylene glycol after entrapment modification by C18E8, with only very slight changes of membrane pore and polymer structures. Correspondingly, PP membrane surface hydrophilicity and antifouling performance were evidently improved. It was also found that the entrapped modifier has a tendency to leach out of the PP membrane in water at room temperature. However, after 8 weeks changes became very small, and the modified PP membrane surface still exhibited significant hydrophilicity and antifouling properties. © 2009 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.memsci.2009.11.062
  • 2010 • 17 Formation, binding, and stability of O-Ag-CO2-Ag-O compounds on Ag(100) investigated by low temperature scanning tunneling microscopy and manipulation
    Hsieh, M.-F. and Li, H.-D. and Lin, D.-S. and Morgenstern, K.
    Journal of Physical Chemistry C 114 14173-14179 (2010)
    The understanding of reaction intermediates in heterogeneous catalysis has important implications for the design of novel catalysts. We investigate the adsorption of CO2 on oxygen precovered Ag(100) at low temperature (17 K) by scanning tunneling microscopy and inelastic electron tunneling manipulation at 5 K. On the terraces, the adsorption leads to O-Ag-CO 2-Ag-O compounds with reduced binding of the oxygen to the surface as compared to the separately adsorbed molecules. The compound can be either dissociated into a bistable O-Ag-CO2 compound at 1.6 V, dissociated into its constituents at 2.2 V, or reacted at 6.5 V into a species, which we tentatively attribute to CO3. The thus obtained carbon trioxide or carbonate is an intriguing reaction intermediate, because it is not stable in the gas phase. Our detailed study of coadsorbed species outlines a possibility to investigate precursors of reactions that involve the substrate atoms. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp104170b
  • 2010 • 16 Effect of urea and glycerol on the adsorption of ribonuclease a at the air-water interface
    Hüsecken, A.K. and Evers, F. and Czeslik, C. and Tolan, M.
    Langmuir 26 13429-13435 (2010)
    This study reports on the influence of nonionic cosolvents on the interfacial structure of ribonuclease A (RNase) adsorbed at the air-water interface. We applied X-ray reflectometry to obtain detailed volume fraction profiles of the adsorbed layers and to follow the effect of glycerol and urea on the adsorbate structure as a function of cosolvent concentration. Under all conditions studied, the adsorbed RNase layer maintains its compact shape, and the adsorbed RNase molecules adopt a flat-on orientation at the interface. Both kosmotropic glycerol and chaotropic urea exert profound effects on the adsorbate: The surface excess decreases linearly with glycerol content and is also reduced at low urea concentration. However, at high urea concentration, parts of the adsorbed layer are dehydrated and become exposed to air. The electron density and volume fraction profiles of the adsorbed protein provide clear evidence that these effects are ruled by different mechanisms. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/la102222z
  • 2010 • 15 Towards a high potential biocathode based on direct bioelectrochemistry between horseradish peroxidase and hierarchically structured carbon nanotubes
    Jia, W. and Schwamborn, S. and Jin, C. and Xia, W. and Muhler, M. and Schuhmann, W. and Stoica, L.
    Physical Chemistry Chemical Physics 12 10088-10092 (2010)
    Adsorption of horseradish peroxidase (HRP) on graphite rod electrodes sequentially modified with carbon microfibers (CMF) carrying carbon nanotubes in a hierarchically structured arrangement and finally pyrene hexanoic acid (PHA) for improving hydrophilicity of the electrode surface is the basis for the direct bioelectrocatalytic reduction of H 2O 2 at potentials as high as about +600 mV. The high-potential direct bioelectrocatalytic reduction of H 2O 2 is implying a direct bioelectrochemical communication between the Fe IVO,P + redox state known as compound I. The HRP loading was optimized leading to a current of 800 μA at a potential of 300 mV. © 2010 the Owner Societies.
    view abstractdoi: 10.1039/c0cp00349b
  • 2010 • 14 Probing the reactivity of ZnO and Au/ZnO nanoparticles by methanol adsorption: A TPD and DRIFTS study
    Kähler, K. and Holz, M.C. and Rohe, M. and Strunk, J. and Muhler, M.
    ChemPhysChem 11 2521-2529 (2010)
    The adsorption of methanol on pure ZnO and A--u-decorated ZnO nanoparticles and its thermal decomposition monitored by temperature-programmed desorption (TPD) experiments and by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), both applied under continuous flow conditions in fixed bed reactors, is reported. Two distinguishable methoxy species are formed during methanol adsorption on ZnO differing in the C-O stretching bands. During the subsequent TPD experiments two different H2peaks are observed, indicating the conversion of methoxy into formate species. By applying different heating rates, activation energies of 109 kJmol-1 and 127 kJmol-1 for the selective oxidation of the two methoxy species are derived. Correspondingly, the methoxy decomposition results in two distinguishable formate species, which are identified by the asymmetric and symmetric OCO stretching bands on pure ZnO and Au/ZnO. Based on the decreased intensities of the OH bands during methanol adsorption, which are specific for the various ZnO single crystal surfaces, on the different reactivities of these surfaces, and on the formate FTIR bands observed on ZnO single crystal surfaces, the two methoxy and the corresponding formate species are identified to be adsorbed on the exposed less reactive non-polar ZnO(101̄0) surface and on the highly reactive polar ZnO(0001̄) surface. The simultaneous formation of H2, CO, and CO2 at about 550-600 K during the TPD experiments indicate the decomposition of adsorbed formate species. The CO/CO2 ratio decreases with increasing Au loading, and a broad band due to electronic transitions from donor sites to the conduction band is observed in the DRIFT spectra for the Au-decorated ZnO nanoparticles. Thus, the presence of the Au nanoparticles results in an enhanced reducibility of ZnO facilitating the generation of oxygen vacancies. © 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201000282
  • 2010 • 13 New types of self-organizing interfacial alginate membranes
    Kahner, R. and Waerder, B. and Arslan, H.K. and Rehage, H.
    Colloid and Polymer Science 288 461-468 (2010)
    In this publication we describe a new self-association process, which leads to the formation of ultra-thin alginate layers at the interface between oil and water. The water phase contains a highly dilute solution of sodium alginate. These macromolecules are negatively charged and they are not surface active. The oil phase contains a small concentration of positively charged surfactants. At the interface between oil and water, the cationic surfactants tend to form complexes with the negatively charged alginate polyelectrolytes in the aqueous solutions. This leads to striking adsorption processes of the solved polysaccharide molecules at the oil-water interface. Upon the addition of calcium ions, a cross-linking process sets in and one obtains the thin viscoelastic membranes, which are anchored at the interface between oil and water. The thickness of these membranes is of the order of 0.2 mm. Similar structures can also be formed by solving positively charged Gemini surfactants in the oil phase. In this case, the cationic surfactant molecules induce the adsorption processes of alginate macromolecules, and they also act as cross-linking compounds. In a series of experiments, we measured the surface rheological properties of these ultra-thin alginate membranes. The results of these investigations point to the presence of electrostatically stabilized membranes. Special interest was given to the influence of the guluronate content of the alginates, which is important for the cross-linking mechanism according to the egg-box model. Finally, this article finishes with the discussion of the proposed building mechanisms of these membranes. © Springer-Verlag 2009.
    view abstractdoi: 10.1007/s00396-009-2158-1
  • 2010 • 12 Coverage-dependent adsorption mode of water on Fe3O 4(001): Insights from first principles calculations
    Mulakaluri, N. and Pentcheva, R. and Scheffler, M.
    Journal of Physical Chemistry C 114 11148-11156 (2010)
    Using density functional theory calculations together with an on-site Coulomb repulsion term (GGA+U), we investigate the adsorption of water on Fe3O4(001). Starting from a single water molecule per (√2 × √2)R45° unit cell, we vary the concentration and configuration of water and hydroxyl groups. Isolated water molecules on the clean surface tend to dissociate heterolytically with an OH group adsorbed on top of an octahedral iron and a proton donated to a surface oxygen. Furthermore, oxygen defects are found to promote strongly water dissociation. The released protons bind to distant surface oxygen to minimize the repulsive interaction between the surface OH groups. At higher coverages, the interplay between adsorbate-adsorbate and adsorbate-substrate interactions and the formation of hydrogen bonds between the surface species result in a crossover to a mixed adsorption mode where every second molecule is dissociated. The energetic trends are related to the underlying electronic mechanisms. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/jp100344n
  • 2010 • 11 Sticking polydisperse hydrophobic magnetite nanoparticles to lipid membranes
    Paulus, M. and Degen, P. and Brenner, T. and Tiemeyer, S. and Struth, B. and Tolan, M. and Rehage, H.
    Langmuir 26 15945-15947 (2010)
    The formation of a layer of hydrophobic magnetite (Fe3O 4) nanoparticles stabilized by lauric acid is analyzed by in situ X-ray reflectivity measurements. The data analysis shows that the nanoparticles partially disperse their hydrophobic coating. Consequently, a Langmuir layer was formed by lauric acid molecules that can be compressed into an untilted condensed phase. A majority of the nanoparticles are attached to the Langmuir film integrating lauric acid residue on their surface into the Langmuir film. Hence, the particles at the liquid-gas interface can be identified as so-called Janus beads, which are amphiphilic solids having two sides with different functionality. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/la102882j
  • 2010 • 10 Effects of wetting layer structure on surface phase stability and on indium surface diffusion
    Rosini, M. and Kratzer, P. and Magri, R.
    Physica Status Solidi (C) Current Topics in Solid State Physics 7 181-184 (2010)
    We study the effects of surface reconstruction and step formation on the surface phase stability, of an InAs wetting layer on GaAs(001). In particular we focus our attention on the α2 and β2 (2×4) surface reconstructions. The two investigated reconstructions have been shown to be formed at an high In coverage, at the onset of the 2D→3D transition. The analysis of the connection between the step stability and the strain distribution around the step edges leads to the conclusion that the favoured step geometries are those minimising the strain. Finally, In diffusion on the flat reconstructed wetting layers has been investigated.We find: (i) the elements of the surface reconstructions favouring In diffusion; (ii) that In diffusion on these surfaces is strongly anisotropic, favoring the [-110] direction; (iii) that the As surface dimers introduce additional adsorption sites with high barriers for In escape. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssc.200982501
  • 2010 • 9 Hydrogen vibrational modes on graphene and relaxation of the C-H stretch excitation from first-principles calculations
    Sakong, S. and Kratzer, P.
    Journal of Chemical Physics 133 (2010)
    Density functional theory (DFT) calculations are used to determine the vibrational modes of hydrogen adsorbed on graphene in the low-coverage limit. Both the calculated adsorption energy of a H atom of 0.8 eV and calculated C-H stretch vibrational frequency of 2552 cm-1 are unusually low for hydrocarbons, but in agreement with data from electron energy loss spectroscopy on hydrogenated graphite. The clustering of two adsorbed H atoms observed in scanning tunneling microscopy images shows its fingerprint also in our calculated spectra. The energetically preferred adsorption on different sublattices correlates with a blueshift of the C-H stretch vibrational modes in H adatom clusters. The C-H bending modes are calculated to be in the 1100 cm-1 range, resonant with the graphene phonons. Moreover, we use our previously developed methods to calculate the relaxation of the C-H stretch mode via vibration-phonon interaction, using the Born-Oppenheimer surface for all local modes as obtained from the DFT calculations. The total decay rate of the H stretch into other H vibrations, thereby creating or annihilating one graphene phonon, is determined from Fermi's golden rule. Our calculations using the matrix elements derived from DFT calculations show that the lifetime of the H stretch mode on graphene is only several picoseconds, much shorter than on other semiconductor surfaces such as Ge(001) and Si(001). © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3474806
  • 2010 • 8 MBE growth of cubic AlN on 3C-SiC substrate
    Schupp, T. and Rossbach, G. and Schley, P. and Goldhahn, R. and Rp̈ppischer, M. and Esser, N. and Cobet, C. and Lischka, K. and As, D.J.
    Physica Status Solidi (A) Applications and Materials Science 207 1365-1368 (2010)
    We present our recent results on the growth of cubic AlN (001) layers by plasma assisted molecular beam epitaxy (PAMBE) using freestanding 3C-SiC (001) substrate. For high-quality c-AlN layers reflection high-electron energy diffraction (RHEED) patterns in all azimuths show RHEED patterns of the cubic lattice, hexagonal reflections are absent. Highresolution X-ray diffraction (HRXRD) measurements confirm the cubic structure of the c-AlN layers with a lattice parameter of 4.373Å. Atomic force microscopy (AFM) scans show an atomically smooth surface with a roughness of 0.2nm RMS. Ellipsometry studies yield the dielectric function (DF) of c-AlN from 1 to 10eV. The direct gap is determined with 5.93eV at room temperature, while the indirect one is below 5.3 eV (onset of adsorption). The high-energy part of the DF is dominated by two transitions at 7.20 and 7.95 eV attributed to critical points of the band structure. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.200983437
  • 2010 • 7 Dissociation of oxygen on Ag(100) induced by inelastic electron tunneling
    Sprodowski, C. and Mehlhorn, M. and Morgenstern, K.
    Journal of Physics Condensed Matter 22 (2010)
    Scanning tunneling microscopy (STM) is used to study the dissociation of molecular oxygen on Ag(100) induced by inelastic electron tunneling (IET) at 5 K. This dissociation is possible above 3.3 V with a yield of (3.63 ± 0.47) × 10-9 per electron. Dissociation leads to three different types of hot atom motion: lateral motion, a cannon ball mechanism, and abstractive dissociation. Analysis of the I -t characteristics during dissociation suggests that the dissociation is proceeded by an adsorption site change. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/22/26/264005
  • 2010 • 6 Effect of surface charge distribution on the adsorption orientation of proteins to lipid monolayers
    Tiemeyer, S. and Paulus, M. and Tolan, M.
    Langmuir 26 14064-14067 (2010)
    The adsorption orientation of the proteins lysozyme and ribonuclease A (RNase A) to a neutral 1,2-dipalmitoyl-snglycero- 3-phosphocholine (DPPC) and a negatively charged stearic acid lipid film was investigated by means of X-ray reflectivity. Both proteins adsorbed to the negatively charged lipid monolayer, whereas at the neutral monolayer, no adsorption was observed. For acquiring comprehensive information on the proteins' adsorption, X-ray reflectivity data were combined with electron densities obtained from crystallographic data. With this method, it is possible to determine the orientation of adsorbed proteins in solution underneath lipid monolayers. While RNase A specifically coupled with its positively charged active site to the negatively charged lipid monolayer, lysozyme prefers an orientation with its long axis parallel to the Langmuir film. In comparison to the electrostatic maps of the proteins, our results can be explained by the discriminative surface charge distribution of lysozyme and RNase A. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/la102616h
  • 2010 • 5 Theoretical investigation of the influence of isotope mass on chemicurrents during adsorption of H on K(110)
    Timmer, M. and Kratzer, P.
    Surface Science 604 1452-1458 (2010)
    Using our recently developed method for calculation of electron-hole (e-h) spectra in adsorption on metal surfaces [Phys. Rev. B 19:165407, 2009], we investigate the system H/K(110). Comparing to our previous results for H/Al(111), we show that the narrower conduction band of K in contrast to Al leads to notable differences in the excitation spectra of electrons and holes. We also find that our results do not obey the scaling of the number of excited charge carriers above a certain energy barrier with the particle's velocity, which is in our case mainly depending on the isotope mass. Instead, we find a different (approximately m1/6 rather than m1/2) scaling. Extrapolating our results to adsorbates with large masses, we expect larger electronic excitations than from the "forced oscillator" approach. This makes the electronic dissipation channel for energy more important even for heavy adsorbates. Our results are in qualitative agreement with other theoretical and experimental results. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2010.05.008
  • 2010 • 4 Co-adsorption processes, kinetics and quantum mechanical modelling of nanofilm semiconductor gas sensors
    Velasco-Vélez, J.-J. and Kunze, U. and Haas, T. and Doll, T.
    Physica Status Solidi (A) Applications and Materials Science 207 924-929 (2010)
    A quantum mechanical model of co-adsorption on semiconductor surfaces is developed and successfully adopted towards exposure to several gases. It is related to nanofilms and thus allows the application of electric fields altering the electronic surface properties of adsorption centres (electroadsorptive effect, EAE). The model is matched against experimental data with O 2, NO 2 and CO measurements under the hypothesis of no direct interaction among the species. However the sequence of adsorption plays an important role where the adsorption of one gas species is opening up other sites that are filled by another sort of impinging molecules. Quantum mechanical modelling of co-adsorption: (a) NO 2 and CO present at the SnO 2 surface. (b) Simplified model. (c) Probability of adsorption. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssa.200983322
  • 2010 • 3 Synthesis, characterization and structure analysis of UZM-22, a MEI-type zeolite framework structure
    Wang, G. and Marler, B. and Gies, H. and Fyfe, C.A. and Sidhu, P. and Yilmaz, B. and Müller, U.
    Microporous and Mesoporous Materials 132 43-53 (2010)
    ZSM-18 related zeolite UZM-22, framework type MEI, was synthesized with choline as structure directing agent (SDA). Sr2+ and Li+ are additional essential components in the synthesis. Rietveld structure analysis of the as synthesized sample confirmed the proposed crystal structure in space group P63/m with a = 13.1544(7) and c = 15.7409(3) and composition (Sr1.17Li0.05)[Si27.92Al6.08 O68]*3.69SDA*23.85H2O. Calcination showed that the material is stable up to at least 600 °C. Solid state NMR experiments revealed that the framework de-aluminated upon calcination and that Li is part of the as-made material as cation. N2 adsorption experiments confirmed the low framework density with a high surface area of 592 m2/g. The 12R pore has a diameter of 6.9 Å. Ammonium TPD revealed a high number of acid sites. © 2009 Elsevier Inc. All rights reserved.
    view abstractdoi: 10.1016/j.micromeso.2009.08.021
  • 2010 • 2 Proton transfer to charged platinum electrodes. A molecular dynamics trajectory study
    Wilhelm, F. and Schmickler, W. and Spohr, E.
    Journal of Physics Condensed Matter 22 (2010)
    A recently developed empirical valence bond(EVB) model for proton transfer on Pt(111) electrodes (Wilhelm et al 2008 J. Phys. Chem. C 112 10814) has been applied in molecular dynamics(MD) simulations of a water film in contact with a charged Pt surface. A total of seven negative surface charge densities σ between - 7.5 and - 18.9νCcm-2 were investigated. For each value of σ, between 30 and 84 initial conditions of a solvated proton within a water slab were sampled, and the trajectories were integrated until discharge of a proton occurred on the charged surfaces. We have calculated the mean rates for discharge and for adsorption of solvated protons within the adsorbed water layer in contact with the metal electrode as a function of surface charge density. For the less negative values of σ we observe a Tafel-like exponential increase of discharge rate with decreasing σ. At the more negative values this exponential increase levels off and the discharge process is apparently transport limited. Mechanistically, the Tafel regime corresponds to a stepwise proton transfer: first, a proton is transferred from the bulk into the contact water layer, which is followed by transfer of a proton to the charged surface and concomitant discharge. At the more negative surface charge densities the proton transfer into the contact water layer and the transfer of another proton to the surface and its discharge occur almost simultaneously. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/22/17/175001
  • 2010 • 1 Anti-nonspecific protein adsorption properties of biomimetic glycocalyx-like glycopolymer layers: Effects of glycopolymer chain density and protein size
    Yang, Q. and Kaul, C. and Ulbricht, M.
    Langmuir 26 5746-5752 (2010)
    In many cases, biomaterials surfaces are desired to be resistant to protein adsorption. A system fulfilling this task in nature is the so-called glycocalyx. The glycocalyx is an outer layer on the cell membrane with bound glycoproteins and glycolipids, exposing a pattern of carbohydrate groups. There is a growing interest to mimic this glycocalyx layer to have a tool to overcome the problems with uncontrolled protein adsorption on biomaterials. In this work a glycocalyx-like layer is artificially imitated by surface-initiated atom transfer radical polymerization (ATRP) of a glycomonomer, d-gluconamidoethyl methacrylate (GAMA), from a mixed self-assembled monolayer (SAM) of an ATRP initiator-immobilized hydroxyl-terminated thiol and a methyl-terminated thiol as diluent. Fourier transform infrared spectroscopy (FT/IR-ATR), contact angle, and ellipsometry measurements were employed to confirm the grafting of the glycopolymer. The anti-nonspecific protein binding properties of this glycopolymer layer were then investigated with surface plasmon resonance (SPR). Three proteins with different size, lysozyme, bovine serum albumin (BSA), and fibrinogen were used as model solutes to investigate the influence of protein size on the protein resistance behavior. The glycopolymer chain density was controlled during surface-initiated ATRP by varying the ratio of the components in the mixed SAM, and the chain length was adjusted by ATRP time. The effect of chain density in combination with the protein size was also evaluated. The most important results are that poly(GAMA) layers of higher grafting density show resistance to adsorption of the model proteins used in this work and that the amount of adsorbed protein depends on the length and density of the glycopolymer chains and also on the size of the proteins. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/la903895q